There are waiting lists in place for existing crew to change fleets (WW to EF and LGW to LHR and vice versa...).
The agreements (also in place with the unions) are that the waiting lists are exhausted before before new entrants are taken on. This agreement, for whatever reason is being 'excused' at the moment.
So, if you have been lucky enough to join BA on a temporary contract, when discussing your wishes to be made permanent, please be considerate to existing permanent crew who are still on waiting lists.