Engine sales used to be lucrative, quite a few companies have been based solely on that business model for years. But just like most things, the market changed over the years. High quality alternatives started to appear, big publishers started making or buying their own tech, and easier entry to market are among some of the reasons.
In my mind, the biggest contributor is simply competition.
This is a very good point. In all honesty, id Software really moved from 'making games' to 'making tech demos' with Quake 2 and Quake 3: Arena. The same is pretty much true for Epic and the Unreal Tournament games.
In my mind, the biggest contributor is simply competition.