Grounds now have to bid for tests which puts the likes of Trent Bridge at a disadvantage because its both a relatively small ground and doesn't have the same hospitality opportunities as the London grounds in particular.Historically the second Ashes test was always at Lord's. Before nuffy grounds like Southampton, Cardiff and Durham became a thing, the standard sort of six test program was
Trent Bridge
Lord's
Headingley
England's Brisbane
Old Trafford
The Oval
Then when they initially went back to 5 tests it seemed like OT missed out at times and it was more
England's Brisbane
Lord's
Trent Bridge
Headingley
The Oval
This series I gather Nottingham misses out and OT is in.
The Welsh Gov used to put cash into the Cardiff bids for Ashes tests as it generates alot of cash for the area, but that doesn't happen any longer so Cardiff wont be holding tests anytime soon.
As ever it's cash and not cricketing reasons that decides the venues.