My guess is they want to run it in London (I remember last time in Wales there were issues with hotels, transport etc. Cardiff is quite a small city compared the others) London obviously has more transport routes and more accessible. The stadium issue could just be down to availability. Wembley might be booked or being used (maybe too big for what they want). But on the flip side in June the Football season is over but this year it might be extended cause of the WC not actually checked.
The transport issues weren't too bad. Last train for most people was before Roman/Drew but that's been standard Cardiff for a while, and ongoing rail worker strikes aren't gonna help that.
Hotels sold out super fast. I was lucky to live relatively close and get a lift back, but even that took us an hour to get out of the car park.
The plus side was the city centre was an easily walkable hub of nutty wrestling fans, so the atmosphere was superb. Not sure it will be as straightforward with London but they should have the transport links to cope.
That said, I'm registering my interest at least. London is no where near as convenient for me and will likely involve an overnight stay, but there'll be more options in the capital at least.