I can remember downloading a demo of Skate onto my xbox's hard drive in 8th grade (around 2007) when the Skate series was first spawned. I was instantly being hooked on the game. I picked up the game immediately. Many of my friends also had the game and we would play (even if it was just a free skate) together online. It was the shit. We were also all into skateboarding back then so it couldn't have happened at a better time
Then skate 2 came out and you could get off your board. and skate 3 just really polished up the physics of the game altogether(although i thought the map was the worst yet)
I guess that they don't have enough ways to improve the game to warrant a "skate 4" or maybe EA simply doesn't have the budget for it.
They should release a version of skate 3 with the same engine just a new map/storyline; A skate 3.5 if you will