The GSA "just worked". Also having the name recognition for public search did a lot to drive its enterprise popularity. It didn't have a lot of bells/whistles, but it addressed the commodity search problem well.

Agreed relevancy is a problem. PageRank works well for public content but internal search has plenty of relevancy problems. Having little control over this certainly hurt.

Managing on-prem hardware/appliances is a difficult business. I don't fault them for moving to a more scaleable model.