Fat bikes were a fashion, and once the fashion was gone, so were the bikes. Sure, there's a niche where they are the best choice (snow and sand), but this is such a reduced field that interest has nearly disappeared.
Actually, it's easy to see when a new bike trend will become a fashion to disappear or is coming to stay. If it improves the practicality or versatility of what's currently available, it will stay. Otherwise, passing fad.
-In the 80s and mostly 90s: MTBs. They would let you go where existing bikes could not go. Here to stay.
-2000s: fixies. They looked cool, highly customizable, etc. but couldn't do anything better than existing bikes. Out they went.
-2010's: fat bikes, great to use in sand and snow, where 3% of the world population live. A pain everywhere else. Out they went. Plus bikes were offered as a happy medium, but people quickly realised they brought most of the issues of fat bikes (heavy, slow rolling) without much of the advantages
-2020s: gravel bikes. They look "fast" like their road sisters but you can actually ride on many more surfaces, so they improve the existing offerings. Here to stay...
-2030s: don't know, maybe 28.5" MTBs??
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bikes" though and apparantly any bike that isn't in one of the bother sub niches is a gravel bike. Low travel flat bar, that's a gravel
