Part of the problem is the use of Sphinx and its search engine (at least this was the case a year ago), where searching happens client-side in JavaScript. This limits how complex the search engine can be, before it is to slow or require to much data to download. Last time I looked it had a simple inverse index with a Porter stemmer and limited ranking of results.

This is great for documentation of most projects as it can be supported with only html+css+js, without a need for a web framework and hosting. But for the documentation of a major programming language I think something more advanced is expected.