The reason Motorstorm 2 failed was mostly gameplay imo. It was a dumbed down version of the first so anyone could play it, which unfortunately is what occurs with most sequels. I loved the first motorstorm, I even won a $1000 tournament. I played it all the time. Unfortunately the sequel plays like Burnout Paradise. The driving mechanics seem very dumbed down. It doesn't even feel like you are off road, and the turning is just long smooth turns. The cars don't flip due landing improperly on uneven terrain, it is like you are driving on a smooth curved road. The cars drive much more cartoony, with very poor physics.
In the first one, driving these light, top heavy vehicles, at top speed, making hard turns, were unstable and it took a lot of practice to learn the tracks, the turns, and the cars, in order to be successful. This unfortunately wasn't the case in Motorstorm 2. You may not agree, but I believe a lot of people feeling as I do, are the reason this game was not successful. For about a year, motorstorm was all my friend and I played, but for this reason, we stopped playing Motorstorm 2, in a day, and returned it.