Usually a combination of less usage and general interest, fewer developers and development time, less or no funding, etc.
Details depend on the project, of course. The lesser used WolfSSL is also FIPS verified; it's clearly not impossible to do these things, it just puts additional pressure on what are often already constrained resources. I mean, the amount of general resources Linux has available compared to, say, OpenBSD is just huge.
I'm not sure that this is really proven out to be a generally true concept. Boringssl is a good example of a very well funded project. There are FIPS modes in the Linux kernel for various things like RNG.
For example Go has a boringssl build because boringssl is FIPS certified whereas the default Go crypto stuff isn't, and this matters for some people.