Well, this started with the PS1 because it was actually quite easy to develop for. It had good tools, the tools were cheap, and you could build your game in C.
Contrast this to the N64 at the time, which had a $1000000 buy in for a developer license, or the Saturn which was, by all accounts, a nightmare to develop for that made the PS2 look like child's play.
After that, the support comes down to the economics of numbers. Most devs I know would have gladly made games on Dreamcast forever, but (piracy/marketing/apathy) killed it, and the PS2 was all that was left.