Simply put, limited history/experience. That's not a bad thing. Rediscovery is good, and often coming at things new, with ideas not weighed down by past thinking can allow for breakthroughs.
> ...a stack language that evaluates right-to-left?
APL, J, K, etc.
> ...a stack language that allows parentheses as a sanity check?
Factor. Not really parentheses, but there are stack languages that use the stack notation ( x -- x x ) to verify stack balance and some that even use the stack notation to validate types. Types and stack balance can even be inferred.
> ...a Lisp that can drop nested parentheses when the meaning is clear from context?
REBOL and now Red, et al. But, as @mananaysiempre stated in another comment, the primary use of parentheses in Lisp is due to functions that allow for variable arity.