...and why now and not 2 years ago...
To be fair to Adobe, even Apple's PRIVATE tools--and the iPhone OS itself--were brand new and nowhere near complete two years ago. It's a young and rapidly-changing platform, and it's clear that Apple pushed hard to deliver the SDK even a year later, and still had things (push, copy-paste, GPS heading) that weren't ready on launching the SDK.
So Adobe didn't have the tools to do this 2 years ago. Nor would it have made any sense for Apple to wait and not launch the iPhone until Adobe finished Flash. It was a chicken and egg situation, and last year--a fast growing iPhone user base but no official SDK yet--was the chicken! We only just recently got the egg.