Another reason I thought for it was; Avatar was just such an astounding show, anything is pretty much messy in comparison. Avatar simply set such high expectations and was hard to out do or even match. However this isn't an option as I feel most would choose it and defeat the purpose.
Also let's not even think about how Nick screwed them over with no promotion, budget cuts, and taking Korra off air.

That said I think part of the reason it was a let down for some is because it was originally only supposed to be a mini-12 episode spin-off. I think Bryke was caught off guard when the demand for more became popular. Seeing as they didn't expected it I don't think they put in the planning they did for Avatar, simply because they didn't think the series was going to go as far as it had. That said with only 12 episodes it's hard not to rushing things; most series I know have 21 episodes to utilize, which is quite a significant amount more and you can do a lot more with it in turn. I understand making the most of what screen time is in the cards, but it can still be difficult. I think that if Bryke knew that Korra was going to make it out longer they would have put more energy into it.

More so though I think it didn't do as well because a new plot was made every season rather than sticking to and developing the old stuff. New plot every season is just one more T.V style (like new plot every episode--like Fairly Odd Parents--or one big theme spanning out for seasons--like Lost). Winx this new plot every season method, and it works for the show. Avatar on the other hand had one big plot; get Ozai, stop the Fire Nation, and bring balance. I personally like that formula better. I feel like those kinds of shows simply do better. I think if Korra had a formula like in OuaT where there is a new theme for each season but the same villains are still in the picture and some themes carry over, it would have done better. One of the things that really seemed to bug the fandom was that (for example) once season 1 was over the Equalist movement was pretty much out of the picture completely and was maybe bought up once when Korra was going through truma--but there was no sign of Equalists still being around. Like-wise the Red Lotus...they covered that up with the time skip. My point is; the one story format seems to sit better with viewers and took well with Avatar fans as that's what made Avatar awesome. With Korra you got attached to the bad guy and he/she died or wasn't really mentioned again. Where as with that one plot formula, the villain (or maybe even protagonist) you grew attached to is still in the picture. Plus a uni-plot show just seems less messy.

With only 12 episodes they always leave out so many important things!

It's like trying to put all the details in the HP books into the movies.

I definitely don't feel like they gave each plot the finesse it deserved. They never expanded on any plot line, even those with a lot of potential to be great. They constantly had new ideas being thrown all over, yet they didn't bother to fully execute them to their fullest potential. They "magically" resolved all of the issues from each book.

In Book 1, we've got Amon leading a revolution against the oppression of non benders. This plot was golden, yet they didn't show to the fullest that the benders were oppressive to most non benders, and they never portrayed why the non benders felt the oppression was too much NOW. And then by Book 2, no one speaks out against oppression because a President is elected and all "issues pertaining to non benders" are resolved and never spoken of again. Also, Korra, the avatar who is supposed to protect equality and balance for EVERYONE, single handedly crushed an equal rights movement and favors one side..... it's like one big contradiction. After the finale, she no longer holds any empathy for non benders, and throughout the show, the stereotype that non benders are not as powerful as benders is made evident, considering most of our non bender characters were never given the opportunity to shine on their own.
And then Book 2, we've got spirits and a new form of bending, and they end it with Korra doing the exact same thing as the villain and leaving the portals open because "let's have unity."
And then in Book 3, air benders are back...... which Korra takes credit for...... but really it's due to the spirit portals being left open, which was technically NOT KORRA'S IDEA. And because air bending is being cast on random people, Zaheer gets it and is now leading an anarchist rebellion against world leaders.
And then in Book 4, we're living through the aftermath of anarchy taking its toll on the earth kingdom, and we're not supposed to sympathize with the "dictator" that actually was the only one bothering to aid her people.

The problem with this show, aside from poorly executed writing, is that the plot lines are always different and because they are not fleshed out fully, they lack and lead this show down a negative path. All of the plots are somewhat tied to each other, but in order to do so, they have to resolve all past issues from the previous book to push the new plot down our throats. And in this process, they've only aided Korra's character in a constant repetition of things working out for her, despite them not being planned or not the original outcome that was expected. This poor execution in plots dragged Korra's character down, and it was a constant repeat of Korra trying to help, gets knocked down, attempts to grow, changes her view for an episode, has a final fight where she resorts to her old tactics, and then wins because she's the avatar...... but is knocked down in the end, only to recoup in the next book. And the cycle continues for the series.

Whether or not it was originally intended to be a miniseries is irrelevant; they knew they were getting continuation before they'd finalized Book 1, and altered the script to reflect that.

It remains that they shot themselves in the collective foot with the short-arc format (and its resultant messily-resolved or dropped subplots, hanging loose ends, and squandered characters). And, on a rather worrisome note: despite all of the above, they've kinda congratulated themselves over said format.

And, on a rather worrisome note: despite all of the above, they've kinda congratulated themselves over said format.

Kind of troubles me too, because I think the long-arc format just worked a lot better for the world of Avatar and the kind of plot-lines Bryke seem to like putting out. I feel like the pair works a lot better with long-arcs than the short ones, I feel like they know how to do long-arcs more. In general I just prefer long-arc showers for stories like Avatar.

Now if they're priding themselves on trying something new and still managing to create a show beloved by many, then I can see why they're congratulating themselves. I mean trying something new and succeeding (or at least mostly so) is something to be proud of.