While I myself don’t care for the change, my idea is this:
Apple now has a very robust environment in terms of first-party, Bluetooth-connected devices. As the Watch, Air Pods, Beats X, and soon HomePod grow in popularity, many people are relying on Bluetooth even more than usual.
Apple, trying to cater to buyers of its accessory devices, changed the behavior to make their lives easier. In their eyes turning off BT completely would be detrimental to the user experience, as many people live a connected life. Of course, it could be a hint that Bluetooth and WiFi radios don’t use nearly the amount of battery that they used to. There is also the argument that WiFi activation aides in location tracking, yadda yadda.
I don’t own any BT devices, so for me the change is largely pointless. Sure, BT may not use much energy, but I have zero use for it presently; for that reason, I prefer to keep it off. No power consumption > little power consumption. Yes, even if that means a few percentage points.
I think these might be the reasons why the changes were introduced.