Because if you show artificially irrelevant results too often (by biasing search results to prioritise AdSense pages), then users would stop using Google Search at some point.
Eventually you would loose the revenues coming from the SERP, which are 10 times higher than AdSense :)
Let's say the search index is size N and the AdSense index is size M. If we were to join them, we would save the storage space for pages that are in both indices.
Also, the search index would gain all the sites that are in the AdSense index so search results would potentially improve. However, it would give an unfair advantage to google publishers.
Would you risk loosing 10, to get 1 ?