I agree it's mostly due to who plays/does not, and also...

where they play. I may be wrong, but it appears they, more often than not, go to the same cities/stadiums.

Why don't they come to Atlanta? I couldn't care less about MLS, but ATL is rabid mad about ATL United, and Mercedes Benz Stadium is probably one of the best stadiums (better than NRG), synthetic pitch aside.

I'd pay top dollars just to see Manchester United no matter who they trot out there (heck, I flew to Houston two years ago just to see them), but I wouldn't continue doing so unless they play their stars or new transfers.