The difference is Oxford had the crowds due to the feel good factor of a club returning from the ashes. Birmingham have been on a downward trend for the last few years. No doubt their fans will be grateful to have some speedway to watch but there just isn't enough of them, there wasn't enough of them at CL level they'll no doubt get some Wolves punters but will that be enough?
They've only gone PL due to the self administered straight jacket that British Speedway has put in itself in with race nights. Surely some kind of special dispensation could have been allowed to enable Birmingham to compete in the CL. No, instead we've got the unbelievable situation of Oxford having (?) to run in 3 leagues, I presume, to act as an insurance to keep the PL at 6 teams, should Birmingham go under midway through the season, to fulfil the TV contract obligations... I can think of no other reason for Oxford to be running 3 leagues?!?