With a d10 you're equally as likely to roll a 1 as you are a 10. So the dice give the game a lot of unpredictability and a range of results all just as likely to come up as another.

Rolling a 2d6, the most likely result is a 7. This is followed by 6 and 8, then 5 and 9, then 4 and 10, then 3 and 11. And the extremes (rolling 2 or 12) are even less likely. So as you can predict the dice roll its easier to predict results and therefore build the right difficulty levels on target numbers for your game.

When I ran Unisystem, I used 2d6 rather than 1d10 but I also tweaked the game's difficulty target numbers.

It all depends on how you want to play your game though, really.