This is exactly the problem, imo. Lua is a niche language because its benefits only apply to a certain niche, i.e., scripting glue on top of an extant system, because that's where the time investment makes sense.
Is Lua so much better that using it is worth losing the convenience of something like Rails or Django? Those things come all bundled up nicely with everything you need for a website. Is it worth the investment to go around and scrape all of these pieces together in Lua? Is Lua going to provide a benefit commensurate with the time you invest in piecing all of that together yourself, including the real possibility that you'll have to write much of the functionality yourself? Or is it simply cheaper and smarter to use Python or Ruby where the frameworks are already composed and widely tested and deployed from the get-go? There's your answer.
To the extent that one wants to increase adoption of a language in a certain niche (say, web programming), you need to at least attempt to make it basically competitive. However cool and fast Lua may be, for most people, it's not worth the investment to start from scratch on a framework when Python has Pylons, Django, Pinax, and Ruby has Rails, etc.