There are a few differences:
1 - Qatar aviation market is vastly dominated by QR. Few international carriers fly to Doha, which means passengers have less options and are mostly dependent on Qatar Airways. Also, there's only one gateway, which is Doha, whereas in places like Russia/Brazil/South Africa people could fly through different places like SVO/DME/VKO/LED (Russia), GRU/GIG/POA/SSA/REC/FOR/MAO (Brazil) and JNB/CPT/DUR (South Africa).
2 - Except for fans from Saudi Arabia, driving is not an option. Trains don't exist.
3 - The World Cup has a strong significance for the State of Qatar and for Qatar Airways. They want their flag/brand to be well represented. Having fans complaining on social media they can't get to Doha because there are no flights available is not desirable. This is basically a once in a lifetime opportunity. They certainly don't want to let it go to waste.
4 - You can be certain international carriers will operate a bunch of charter flights to Doha. BA, Air France, Lufthansa and the likes will show their faces at DOH bringing fans from their respective countries. Still, QR will be the main carrier and provide the core of seats on offer for the event.