One thing immediately comes to mind for me. Dexterity. The game "Daxter" for the PSP is one such example. One dexterity level after another. Hey! I'm just not that good with the thumby thingys on any system which has them. Give me a mouse with two buttons and a scroll wheel. "Daxter"...shelved.
"Final Fantasy II". Go a few steps, fight, go a few steps, fight. Tedious. I did finish it though. But I sure can't stomach a second go at that tedious game. Shelved.
"Doom 3". Reach a certain spot, (same spot every time), click, out go lights, fight creature you can't see, too dark. "Doom 3" ...shelved.
"Blood". Blue screen of death, Blue screen of death, Blue screen of death, Blue screen of death, Blue screen of death. "Blood"...shelved.
Many different reasons, many different games.