I would imagine the MOD got some sort of bulk purchase price for the winches.
It's also probably a logistics thing...easier to move a 2 drum winch about (back of a Landrover?) especially for servicing / re siting etc than a 6 drum thing.
Possible thinking is also that multiple 2 drum winches is better than 1 x 6 drum winch if you have an engine problem on one winch...it doesn't ground you!