Bribes, some of which came out and are currently under investigation/prosecution.
Other reasons why it's a terrible idea: they have a small population, little football culture (in terms of absolute size), no infrastructure (all the stadia are newly built and some will be demolished afterwards; there aren't enough hotels); they have many laws incompatible with what usually happens at World Cups. There literally isn't a single positive thing to be said about Qatar hosting the World Cup.
That makes it a great airport hub, but it doesn't have the infrastructure to host, feed, transport, place in a stadium the hundreds of thousands of fans that want to watch a World Cup.
Other reasons why it's a terrible idea: they have a small population, little football culture (in terms of absolute size), no infrastructure (all the stadia are newly built and some will be demolished afterwards; there aren't enough hotels); they have many laws incompatible with what usually happens at World Cups. There literally isn't a single positive thing to be said about Qatar hosting the World Cup.