It's more that it's easier to use C for two reasons. The first is that C is really popular and therefore pretty portable. It's a lingua franca. The other is that because the hosts are largely defined in C, it's easier to interact with. Of course, the host doesn't actually "speak C", it follows some form of ABI. But the reality is that implementing each ABI is non-trivial and you can avoid a lot of pain by just using the host's C compiler/linker/etc. that implements it for you.
Like, SBCL compiles and assembles directly to machine code, that's very much the Lisp way. But SBCL has a lot of C that's involved in getting the SBCL image running and interacting with the host.
