Why doesn't Common Lisp have continuations?

Continuations are a great theoretical tool; if a language has first-class, multiply invocable continuations then one can build threads, exceptions, coroutines, and the kitchen sink on top. However, there is an implementation burden with continuations; supporting first-class, multiply invocable continuations complicates things tremendously for the Lisp implementor. The ANSI standardizing committee J13, mindful of this, took the view that it would be better to specify the user-level control structure (CATCH, UNWIND-PROTECT, and so on) and let implementors choose whether to build those on top of continuations or not.