I actually think that the problem is in reverse. Way I see it, Sora did develop. He had some serious growing up to do in KH1, which resulted in some ridiculous anger issues in CoM. The mind-wipe messed him up to make the bad writing in KH2 remotely bearable but all of it was still there - manning up to the duty of being a Keyblade Wielder, as well as the suppressed frustration and anger that surfaced like in Hollow Bastion.
One of the things I did like in both KH1 and KH2 Sora, that KH2 Sora admitted out right - his happy-go-lucky is a mask, a pretending, but he really is trying his hardest for everyone. As opposed to many characters, however, his angst wasn't a given - it was something he acquired along the way and I appreciate that the most.
But the point comes when you deal with his plot connections. When we deal with the KH1-CoM-KH2 trio, Sora was, in a sense, plot-redundant. He was the kid with the Keyblade and that's why everyone and their mothers were after his hide. However, that's where everyone's plans fell apart, much like how AtW said himself - Sora's own personality and actions came into play, and it's who Sora was, rather than what, that mattered.
Standing contrast to him was the lackluster Kairi whose what was basically everything she had, be it the PoH or "the girl Sora cares for", whereas Riku, while leaning closer to Sora, was the only one of the three who had any semblance of a decent plot establishment due to his given role of falling into the Darkness. So we got him moving the plot along with being developed.
Sora only had being developed while the plot pushed him forward.
However, that's where Days and BBS came in. Sora wasn't the playable protagonist. But it's Sora's story so they had to tie it all back to him somehow again. So we have a game focusing around his Nobody and Replica, and BBS had Terra become the main antagonist, Ven be absorbed into Sora, and Aqua be some distorted female Sora with self esteem issues who went to hell.
Sora to begin with wasn't a plot device, and so long as the whole "KH = Sora's story" bit was believable, he didn't need to be. But the series has expanded far too far, and somehow, they had to tie it back to Sora, thus turning over half the cast to characters whose essences was to feed Sora plot to justify him being the protagonist in future installments. :V