The only 'truthful' answer I've ever been able to come up with why the BT/EE merger was allowed and the O2/Three merger was rejected was the lining of pockets.
It certainly wasn't down to European 'competitive landscape' reasons, EE/BT + Vodafone + O2/Three with equal spectrum would have been great for competition because the playing field would have been level.
That is unless that is Ofcom were convinced they would collude to jack up prices in step with each other... which might be a second 'truthful' answer, but as you say we'll never know.