There's no silver roadblock preventing Lisp from becoming popular. The problem is historical, and the only way to over come it is by brute force or by some random fluke like some new super-popular app is only scriptable with Lisp.

Complexity of implementation? Slowness? Lack of marketing? Unfamiliar syntax?

The answer is more like 'all of the above,' as well as not anticipating the rise of Windows and Unix (which is not necessarily a bad thing).