Well, I know this soalan was asked half a tahun ago, but I wanna answer it anyway. Actually Disney wanted Giselle to be a "official" Princess too, but since she looks like Amy Adams they would had to pay her...So they leaved her out from the Collection.
I would like her to be princess, but the problem is that Disney did not want to pay Amy Adams more. Also, she didn't become a princess at the end of the film so if ever she would be a princess sejak merit like Mulan. However, she is beautiful and deafeated the bad guy at the end so I think she deserves lebih attention.