We all know the reason for these kind of changes is to reduce costs & liability and to enforce control from the GB / WTBTS.
Too many district overseers are "free spirits" who tended to go "off message" in many of their talks whereas Bethelites tend to be a lot more insular & company men.
I remember a brother I knew as a real character when he was younger coming back to visit our congregation & he stayed with our family for a few days. He had gone to Bethel from our congregation and in 20 years had been virtually lobotomised and completely institutionalised and his formerly pretty , lively wife just had that sad far away look in her eyes. She'd developed some kind of nervous illness that made all her hair fall out so she wore a wig. I overheard her telling my mother that she was desperately unhappy and unfulfilled and spent her time cleaning rooms and toilets at Bethel. The rooms were so clean that she & the other wives spent hours standing around in corridors folding & unfolding towels till their shift ended as they weren't allowed to clock off early. It was quite sad really , but not untypical. My Dad used to get a daily newspaper - one evening he asked if the brother wanted to read it but the guy refused to even look at it as he said it was full of "Satan's news from this old system. "
If nothing else , some of the senior Bethelites are reasonably good speakers who can deliver a society outline without deviating and spend most of their time heading a pretty boring department ( eg laundry or shipping. ) It makes sense to utilise these guys a bit more so getting rid of DOs and replacing their talks and their administrative duties with a guy shipped in for the weekend from Bethel serves both purposes.