Not surprising, considering the vast majority of consumers don't need more storage than you can easily fit in an M.2 form factor. Why mess around with expensive enterprise U.2 drives when it gives you zero benefit and gains you the hassle of dealing with extra wires and finding space for an ugly bulky rectangle?
Once you get to storage sizes where 2.5" becomes a need, you are well beyond the consumer price range. Very few people are willing to spend more on a single SSD than on the rest of their gaming computer combined.