As I understand it, BA just don't have enough de-icing capability to run their schedule of departures.
Queues and departure delays waiting for de-icing means aircraft staying on stand, which in turn gives issues for inbound landings with nowhere to go, and the whole thing gets disorganised.
I did see some very impressive custom-built de-icing units BA showed a few years ago, which must have cost a bomb, and doubtless bought a smile to their salesman. However, elsewhere in the world a straightforward truck-mounted bit of kit appears to do the job just as effectively. I would guess you could buy several of these for the price of one of those gold-plated units.