Tony hawk games were the only other skating games, but they had button-press controls and goofy physics.
Skate came out with much more realistic physics and the thumbstick flicking controls that made tricks feel purposeful.
I think people who do skate (or wanted to) got pulled in for the realistic skating lines and tricks. There was also still enough over-the-top jumps and tricks to keep younger kids entertained.
Tons of reasons. My reason is that it felt like the first skate simulator. And the control scheme was freaking brilliant and intuitive. I couldn't adapt to Session... too complicated.