Solaris is more BSD based than AT&T-UNIX based; csh is the historic choice for BSD, and most BSD-descendants honor this, as it is what BSD users expect. For ex

Solaris is more BSD based than AT&T-UNIX based; csh is the historic choice for BSD, and most BSD-descendants honor this, as it is what BSD users expect. For example, it is the default user shell on FreeBSD. But the system shell is a Bourne-like shell (ash, I believe?). So for serious usage, even the modern BSDs defer to a Bourne shell.