It's also easier to find and fix bugs with smaller numbers of people, especially performance bugs which can be amplified at scale. So it gives them a lot of time to work through issues over the beta. It also gives them time to build teams around the expanding infrastructure and build processes for monitoring and handling issues as a larger team.

Plus, these invite only periods start with more tech savy early adopters who more willing to put up with issues, and willing to provide decent bug reports to fix them.