There's a small page on the c2 wiki that asks the question -

As lamented on AllPanaceasBecomePoison, part of what poisons a language is that if it's got any real power it soon grows ExtremelyInterstrangled. Well, political woes aside, that doesn't seem to have happened to lisp and smalltalk. How come?

The first link in the paragraph goes into how and why C++ became "poison" over time - the layers of complexity that got added eventually created this:

Why have languages like C++ (and Java, and C#) gone toward the image above, when Smalltalk and Lisp haven't? Well, both Smalltalk and Lisp are simple languages - Smalltalk has 5 reserved words and two operators. When Smalltalkers need power, they don't need a JSR that adds a new set of keywords and features to the language. When Java developers found they needed generics, it came with a layer of new complexity. When Smalltalkers needed the same thing, they found that unlimited polymorphism already provided it.

The reason Smalltalk hasn't become "poison" is that we haven't spent the last 25 years larding in new reserved words and "features". Instead, we've been writing libraries. Fortunately for Sun (et. al.), most developers really like complexity - so they keep going for the shiny new toys that come pre-packaged with inscrutability