Not sure but I think ... scale.
" In July 2024, running a Relay on ATProto already required 1 terabyte of storage. But more alarmingly, just a four months later in November 2024, running a relay now requires approximately 5 terabytes of storage. That is a nearly 5x increase in just four months."
This is from a great article: https://dustycloud.org/blog/how-decentralized-is-bluesky/ from @cwebber