This seems like only one of SSD standards made by engineers, where you have proper size, proper connection and proper cooling.

Yet U.2 mass implementation had been quickly shut down by leading motherboard manufacturers, in consumer and HEDT segments they limit U.2 ports to one or two intentionally.

With making lot of U.2 ports standard feature you significantly simplify board design compared to any modern advanced board and this can lead to much faster penetration of Chinese competitors. Add to this that U.2 ports make it quite hard to segment motherboards (now MB with 3 NVME slots is considered top one and they charge premium). Also most MB manufacturers are players in the M.2 NGFF SSDs market.

SSD makers are also not fans of 2.5" form factor with proper cooling. They much better sell you small board without any of it.

M.2 NGFF had been designed for fast thin notebooks (With Apple being main driver at the time). And also for blade servers. And not to be bough and used by end users (only static electricity kills around 3-5% of such disks during install).