It also significantly helped that Lua's main users could easily use an older implementation if something they depended on/didn't feel like fixing was removed. I

It also significantly helped that Lua's main users could easily use an older implementation if something they depended on/didn't feel like fixing was removed. In contrast, breaking backwards compatibility for Javascript would be unthinkable. OTOH, there is real fragmentation between 5.1 and 5.2, so not even Lua is immune to the downsides of breaking backwards compatibility.