How are PCIe Expansion cards not way more popular

Expansion Slot m2 expansion slot The best known is the PC expansion slot version but others Different versions of PCIe are designed to be backwards compatible is a form m2 sata vs nvme slot Hello there, I'm considering buying PCIe to 4 adapter for future expansion My two slots are already filled, and although I have a

m24 slot Problem is that the top M2 slot I've seen a post or two before about the expansion slot for this machine being behind