Why did this phone attract so much development? It's not particularly great or anything. I've seen many phones with better specs get much less dev love.
I am no developer--yet, but here is my take on the reasoning on it:
Price point: they were giving them away for a while and have never been more than $100.
No bootloader shenanigans.
Resiliency and durability: Solid as a brick, yet hard to brick.
Longevity: Sprint is still selling them 2 years later.
Large family (P500, Opti V, T, etc.) and wide circulation ensures a good test bed for development.
Great initiative early on: Zefie, KSmithinNY, ath3nos and others got a lot of the important stuff done, paving the way for others to follow.
Less than stellar official updates and support.
And not meant for last, but typed that way:
Android Central community and forums: This is/was a "starter" device for both users and developers; its success stems from these folks working together, providing feedback, support and encouragement. The Optimus S isn't even listed on XDA. Proof that developer talent and positive interactions with other devs and users can work.