I guess the reason is the same as for most of the missing features, with limited amount of time we choose the most requested features and implement those. We'll update the site in the coming months and I'll see if we can get some service where you can vote on the features. Then it will be more transparent.

As for the feature in question, statistics has its own cache which is built in the background. This enables it to work quickly. Autotags pose a problem, since with every rule change all of the cache needs to be rebuilt. So we'd have to find some other way for it to work effectivly with autotags and we never got around to implement it.