Why spend the money for a super fast direct pcie ssd and then pay more money to cripple it down to sata speeds when you could just buy a sata ssd or msata+adapter way cheaper?
Another reason could be size - in a 5.25" form factor, you could potentially "stack" several m2 SSDs in blade-like fashion. I haven't seen such a thing though, and heat might be an issue.
So there are 5.25” multiple m2 adapters out there like
https://www.icydock.com/goods.php?id=255
That one would require a sas controller to connect to.
You generally don’t see a tighter system like blades due to needing a bifurcated pcie slot support, which at that point of server hardware, your are generally not using adapters like this.
Is the ability to bifurcate a PCIe slot (and avoid extra controllers) that rare? That's a shame.
I have a https://www.supermicro.com/en/products/motherboard/X10DRH-iT and they don’t support bifurcated pcie. It’s a shame cause I’ve extended the life of the servers by using pcie adapters to add high speed io layers and I’m limited to one device per port. I’d love to double that density.
Hot swap bays are usually set up for 2.5" drives. You can fit 4-8 into a single 5.25" slot. But the ones designed for performance will prefer U.2 drives and hook them up to PCIe.