From a purely operational standpoint, rapid growth stresses a network service not only in its technical capacity, but also in the ability of the service's operators to keep up with fighting fires. Engineering capacity to work on a service is itself a limiting factor on healthy growth.

If the tools aren't yet there to mitigate a rapid growth in abuse problems, then it just makes sense for them to limit their exposure to the rapid-growth part of the network. It takes time to write those tools.