[oldfart]

Muslims underachieve for the same reason as all other underachievers: Low self-esteem.
Sure, we can say they are stupid or backward or 'stuck in the 8th century,' but all of those things can be changed by anyone who truly believes in his own abilities. Muslims have spent the past few centuries being brainwashed into believing they are superior to everyone else because they live simply, without most of the basic things that make life pleasant here.
Now they look across the ocean and see people living better, more comfortable lives while breaking all the rules they've been taught. So their teachers tell them all that is the work of the devil and their task is to defeat that devil and one way is to wear this belt of C-4 and batteries.
For them to question the reason for the explosive belt would lead them to question all the other things they've been taught and that would open up a whole new can of worms that their brainwashing doesn't allow for.
They are fanatics and, just as gun-control fanatics cannot be reasoned with, they can't see anything except from their own point of view. While it is not politically correct to say so, in the end, we will either have to kill them or convert to Islam.

[The_Reader_David]

Actually you have it backwards. Muslims underachieve because they have too much self-esteem: they fancy they have the true religion so they can rest on their imagined laurels.

Self-esteem is a vice which leads one to complacency (among other bad effects).