Christians send a handfull of missionaries to other countries whereas Muslims flood other countries with millions of true believers/activists. Which approach do you think will get quicker results?
Those countries are HOSTILE and/or closed to Christian missionaries. In many cases, they have to go in disguised as workers of some sort (like Heather and Dana in Afghanistan).
We, on the other hand, open our arms to just about anybody. The blessing and curse of being a democracy.
Another thought ... Christians try to convert others by preaching and through good examples. Muslims spread their cult with the sword.