Q:

Why aren't tag scores updated live?

I'm kind of wondering… why can't there be cached tag scores updated live (e.g. when one of my answers is upvoted), like there are for reputation? I'm not saying that there shouldn't be the script running nightly, but it would be nice to see my scores go up immediately when my posts are upvoted, as opposed to the excruciating wait for 4AM UTC.

A:

like there are for reputation

Reputation score is updated live because it's essential part of Stack Exchange, seen and "used" by each and every user. That's why it's always visible, anywhere.
But keeping it live isn't simple or cheap task, it required development time and consume resources.
Tag score on the other hand is something important, but only for small part of the users. It affects only some badges, and isn't visible in many places, only when one is looking for it. So while it's important to keep it correct, can't see any reason to make it live.

