It's due to serialization. Presently, a consensus node that cares about "can user x spend y amount of STEEM" must process every single post and every single comment, in order, to know if that's possible or not.
Exchanges, for example, only care about your main balance for sending/receiving payments. They would not need to process each and every post and comment.
It's also a matter of parallelism; the balances can be calculated in parallel and only require synchronization when the rewards balance is transferred to your liquid balance, which should be about a 10x reduction in calculations for the liquid balance.