According to some of their sources, one of the major reasons they chose to go with a new protocol is because they wanted to take decentralization a step further by separating user identity from user instance (known in the AT protocol as a Personal Data Server, or PDS)
In other words, they’re trying to make it so that even if the instance you were on were to disappear, or you were to get banned from it (as what can happen on the Fediverse), you wouldn’t lose your identity. You could just use a recovery key to reauthenticate your identity and your followers would be able to find you again. Contrast with the Fediverse where if the instance you are using disappears suddenly, or you are banned for whatever reason (not even necessarily because of bad behavior, but also something possibly like petty disagreements), your whole account is gone and people will not be able to find you unless you create a whole another account. Even in situations that doesn’t involve accounts being deleted, this would also be for if you wanted to move to another instance that might be blocked by the one you’re using
At the moment, the way users are identified in the AT protocol is primarily through a placeholder version of a w3c standard that consists of a bunch of random characters, All of which is contained within a DNS TXT record. Essentially identifying every user either through that sequence of characters alone, or through a friendly domain name separate from your instance like @soulfire.jarexibackblaze.xyz (My personal account handle).
TL;DR Bluesky uses a new standard because they wanted to fix the problem of users losing their identity because their instance went down