Ubuntu chose it as the default shell for sysvinit startup scripts in 2006 (from which it spread to BSD) precisely because it was much faster than bash.  It's also smaller: bash is a memory hog. 

When I wrote a whole (batch) application in about 120 Perl and shell scripts in 1999-2001, I often needed multiple shell scripts running simultaneously, sometimes for concurrency and sometimes just from scripts calling other scripts.  So I made sure everything ran under Solaris sh, which was a modified Bourne shell at that time and so was much lighter than bash, which I used for development.   Nowadays I'd use dash in the same circumstances.