Besides being portable, Base is also intended to be much more lightweight than Core. It has no dependencies other than sexplib0 (which is itself very small), and so it compiles much faster than Core.
In addition, Base has stronger stability guarantees than Core: it promises to be compatible with at least 4 major versions of OCaml. So, for example, 4.08, 4.09, 4.10, and 4.11 in v0.14.1. (Actually, thanks to kit-ty-kate, it also supports 4.12).