As Daxx says, fights in Vegas make sense - but the other reason the fight couldn't happen at Wembley is because without HBO's backing the fight isn't happening.

Primetime HBO PPV would mean the fight starting at 4am UK time.

Wembley has an 11pm deadline for events. So the fight would have to be primetime UK, which is no good for HBO, so no fight.

Just imagine the hassle of dealing with 80,000 drunks in the early hours of the morning .. that's if it's even possible to get those numbers then