Centralized is easy.
That's probably all it comes down to. A fully distributed network is far more complex to design and develop and often infinitely more complex to understand than a centralized one.
For a centralized hive mind, all you need to understand is how the central brain works, and you can predict what it'll do. If it doesn't do that, you know where the problem is coming from and where to look for a fix.
If you need to make some design changes, you only have one place to manage them. All the drones in the network are stupid; they just do what they're told. If the central brain can be taught to deal with the problem, the drones can probably stay as they are. If they need to be changed, they can be tested in isolation in a test chamber.
If you build a new type of drone, you just need to update the central network to be able to command the new drone type and to let it interact with the others, if necessary. The new drone, again, can be built and tested in an isolation chamber; as long as it does the right thing when instructed and feeds the right info back into the central hive, it's going to work perfectly well with everything else.
When you're dealing with a distributed network, nothing is easy. All your drones are self-operating, so it's hard to test them in isolation. Even if you build a full environment for one of them to play around in, all you'll know is that the drone does a certain thing when alone. You have no idea how the drone will act when it encounters a fellow drone, because not only will your drone behave differently now, but the other drone you introduce it to will also behave differently. And each type of drone will probably have a different type of reaction. And having more than one drone at once will also change what is happening. Even different if it's a different combination of drones. It's hard to predict what the various drones will do.
Whenever you want to change a distributed network, you need to not only think about the main network and how to modify it, but you need to think about how all the different drones are going to respond to new instructions.
If you build a new type of drone, not only does the main network need to deal with that, but all the other drones need to learn to interact with it in a useful way. And probably those drones are pretty limited in terms of thinking power, so good luck with that.
Using primarily centralized intelligence, we can drop human beings on the moon with all the computing power of a modern calculator. Using distributed intelligence, we can only now barely produce something capable of navigating a quiet road and we're still terrified of letting it do that around actual people.
Sure, having a fully functional distributed system that perfectly functions will beat a centralized system with absolute ease. But there's a good reason humanity invented the stone hammer tens of thousands of years ago, but the self-swinging-hammer still isn't a thing.