Not happy with the shell quality, code quality, or license. Bash is massive compared to pdksh (what openbsd uses), has frightening code, and openbsd rejects GPL code as much as possible (gcc/binutils is the only GPL software in the tree, and no new GPL software is considered).
I think it is because bash is a huge thing, both in terms of program size (functionality) and attack surface.
Most BSDs don't ship bash as their default shell. FreeBSD, for example, ships tcsh.
BSDs haven't shipped with bash historically and the BSD projects do not like shipping GPL software.
Does OpenBSD ship zsh by default, out of interest?