Why doesn't bash treat brace expansions exactly like csh?

The only difference between bash and csh brace expansion is that

bash requires a brace expression to contain at least one unquoted

comma if it is to be expanded.  Any brace-surrounded word not

containing an unquoted comma is left unchanged by the brace

expansion code.  This affords the greatest degree of sh

compatibility. 



Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.