The same reason it would be a good thing to use JS under any circumstances: there's already a huge supply of developers familiar with it and its ecosystem, it's available on pretty much every platform (now including at least one dedicated unikernel environment), it's a natural fit for web application development, and so on.
It's clear why unikernels and OCaml go together so nicely: immutability is a concept central to them both.
That said, the affordances of using a unikernel seems fairly orthogonal to the choice of language, IMO.
