I would really like to know the reasons, puzzling
A lot of it will come down to handling agent acceptance.
If the BHX agents are busy, or have minimum staff due to not being busy, they themselves can refuse the divert.
BHX are usually quite good at being able to accept diverts, but, cost cutting and undercutting the competition amongst handling agents (which is not just a BHX issue by any stretch of the imagination), means that staffing levels are now close to the bare minimum for the scheduled flying programme, which in turn means less diverts as there is no one to handle them effectively.
Obviously the above is pure speculation as we don't know the reasons diverts are refused/not used, but, the above is happening more and more