The problem with SFU was that it implemented some sort of a generic Unix system - it wasn't Linux, or BSD, or anything else specifically, just something POSIX'ish. So it only had source-level compatibility, not binaries - you had to recompile - and then compatibility only extended to those Unix and POSIX APIs that SFU implemented.
SFL, on the other hand, implements Linux kernel ABI (syscalls and device nodes). Which then allows to just put glibc on top of that, and getting full compatibility with userspace Linux APIs for free; your choice of distro (though only Ubuntu is officially supported).
Since it emulates an actual OS, and does so on binary level, this is much, much more useful than SFU ever was.