Distribution is part of it. They also are... (Blech!) selling a quality product. Think about everything else out there in that class of light American Lager...
I'm not taking a position on this, but I would rather drink Bud than Miller or Coors or PBR... (But I drink none of that stuff.)
Of course everybody has their preference and I'm not suggesting that you can't enjoy your MGD if that's what you like.
But Budweiser (Anheuser Busch) had a lot of early success with the brewery in St Louis and the underground caverns they were able to use for lagering their beer... The business acumen has always been a large part of their success, including the merger to Inbev (Now
AB Inbev... as in... Anheuser Busch).