Because Wayland is fundamentally very different from the older X protocol, and many programs don't even directly do X. They leverage libraries that do it for them. Those libraries are a huge part of the lag. Once GTK and Qt and the like start having a stable Wayland interface, you'll see a huge influx of support.

A big part of the slowness is why Wayland is a thing to begin with. X hid a lot of the display hardware from apps. Things like accessing 3d hardware had to be done with specialized display clients. This was because X is natively a remote display tool. You can use X to have your program show its display somewhere else. Wayland won't do that because that's not the point. Applications that care will have goals for change. Applications don't care will support it once someone else does it for them.

Right now, the only things that would benefit from Wayland are games and apps that make heavy use of certain types of hardware. Half of those don't care about linux, while the other half is OK with X and xwayland.