The issue here is human decisions, not a taxonomy of programming tools. Or do you suggest that Lisp dialects diverge further? If you called them separate languages, would there be a better chance of them growing popular?
On the contrary, I'm suggesting that Lisp dialects do not diverge as far from each other as the various dynamic languages do. That means that the idiosyncrasies each dialect has are going to be more glaring. The other dynamic languages don't (yet) have dialects, so you don't have to take the design decisions of the interpreter into consideration - after you've decided on the language.
In that case, I think that there might be a better chance of a sub-set of Lisp dialects becoming popular if they were described as separate languages. It will be interesting to see how Arc develops from MzScheme, and whether or not it becomes more popular with non-Lispers.
