I've seen similar behavior and the theory I came up with is that managers are engaged/rewarded by being associated with successful projects and technical people

I've seen similar behavior and the theory I came up with is that managers are engaged/rewarded by being associated with successful projects and technical people that report to them are engaged/rewarded by new projects and challenges. I speculated that from their perspective someone tied down to maintaining a very successful project (that continues to make succeed or make money) is in a great position politically and those getting the new green-field projects are being exposed to risk. It reminds me of meetings–managers don't realize that they are a drain on the people performing the work because that's of course how they work and accomplish things.