There seems to be a widely-held belief that corruption has been rife at the top of FIFA for a long time. This corruption supposedly includes the people on the committee which votes on World Cup bids taking effectively personal payouts in order to vote for particular nations to get the World Cup.
I recently watched a documentary about FIFA on Netflix. It wasn't particularly well done and I, of course, don't know which allegations made within it are true and which aren't. My gut tells me it's generally right about FIFA corruption, but I wasn't in the room so to speak so I have little idea just how pervasive it was.
As to your question in particular, the accusation is that Qatar effectively bought the World Cup by effectively making payments to respective voters or arranging business deals which benefited the nations that respective voters represented. The U.S. had, it's claimed, been the heavy favorite to get the 2022 World Cup. The accusation also seems to be that the people in charge took the unusual(?) step of awarding both the 2018 and 2022 World Cups at the same time, in 2010, because they might soon be out of power and they wanted to maximize how much payola they could collect while they still could.
I don't know to what extent those accusations have merit. I don't, however, find them hard to believe. And I would say this, when they interviewed some guy representing Qatar for the World Cup bid and put the accusations to him, his responses rang smarmy to me. He didn't lessen my inclination to believe the accusations.