Moan all you want, it's all about capacity.
To release a current BA A320 pilot from SH to LH involves the LH conversion course and then a SH course for his DEP replacement into SH.
A DEP LH requires a LH conversion course.
One less course in a system where simulators, trainers and capacity are at maximum.
Join when you can, put up with SH, move over to LH and find yourself already further up the list than if you transferred today.
Conversion courses cost A LOT of money and, in the current climate where capacity is severely limited on certain fleets, cost is king over internal bidders I'm afraid. IAG thus BA will go for the cheapest option.