[markstos]

A POSIX-compatible shell like `sh` is a given. `csh` is not POSIX-compatible. By dropping `csh`, the the base system could be smaller. Since the usability of `sh` has improved, it was considered worth switching.

[1500100900]

On top of that, most people expect syntax somewhat similar to Bourne shell and tcsh is far from such. Its for loop syntax is especially surprising.