Google built GSA at a time when there were limited options for search, and when most tech companies had to manage their own hardware, and when it was magical to have any kind of search at all. It made sense at the time.

But now, the problem itself has changed. Companies host on the cloud, there are many options for search (even hosted/managed solutions), and customers' expectations are very high for good search results. All of this boils down to the biggest issue that most companies face re: search - the business logic of providing fast/good search results (rather than maintaining a search engine itself). Also, companies increasingly view search engines as a first class information retrieval system (a good compliment to a source-of-truth database), which can empower internal analytics to make business decisions.

Most companies don't need/want Googly algorithms/hardware to power their search. I'm guessing here, but I bet that recent versions of Solr and Elasticsearch put the last nails in GSA's coffin.