Basically, because they didn't warn Bluesky that they were going live, they had their traffic heavily trottled and they were unable to function.
So long for decentralisation if you need to talk to the owner of the protocol you are trying to use...
The default rate limits are quite reasonable but big launches will blow past them so you gotta coordinate with someone.
And note that the relay federation/gossip agreements are largely manual configuration at the moment. Until someone pushes a proposal for an automated/dynamic topology management algorithm that doesn't get plagued with spam issues it'll stay this way.
You probably just don't notice it as a user or as the operator of a small instance which to be fair is exactly the same with atproto. Users rarely notice rate limits and small operators rarely do either. It's the operators of large services that see these limits and have to coordinate with other operators.
The issue with W social is that they have a chronic inability to coordinate or play nice with anyone else in the atproto space. Rate limits aren't a problem if you are a good neighbor and you know your neighbors but frankly they just don't.