Wouldn't it be better to tie this to rating percentage instead? E.g. the top 10% of users gain access to site analytics, the top 15% are trusted users, etc.
Not really. If you look at all time reputation vs. an all time percentage, reputation is much more stable than a percentage. Imagine there are 1000 users. And 100 of those are spam accounts. One morning, a moderator decides they're bored, and removes the accounts of the spammers. Suddenly, 10 users lose their access to site analytics.
If you look at percentages for shorter time periods, you reach the problem of inactivity Glorfindel describes. I disagree with that answer stating that it may have merit on sites with little activity though: it would mean users can unlock a lot of privileges without first showing an understanding of how the site works.
Also, right now, once you reach a privilege level you can focus on using it. If you have to make sure you're in the X% group to keep using it, this adds a (probably unwanted) focus on keeping your privileges, instead of on curating the site with those privileges.