It was always speculative that it would be the "next big thing". The basic claim of this tech is that a higher bandwidth screen experience is a better experience - and that's always had practical constraints, like, I'm not replying with a 4k video message here, because it's not the medium for that.

Between AR and VR, AR is the harder to address comprehensively - we knew going in that we can make pretty pictures in VR by leveraging existing CG pipelines, and there's definitely some market for aesthetic immersion in VR. But VR has also mostly hit a wall outside of that realm since it gives up a lot to achieve this goal - cost and accessibility are both much weaker relative to 2D screen experiences. It's badly positioned for consumer use despite intensive marketing in that direction. AR adds the extra claim of making use of more of the real world, and this is still a thing with potential, but again has trouble in many cases justifying itself relative to the headaches - for a lot of applications the quality and kinds of sensor data needs to be higher for it to really give results.

Net result: a bunch of startups got funding to chase after the buzzword, the survivors mostly pivoted into more traditional businesses.