I worked with Closure for a couple of years. It's great, but it has none of the characteristics that tend to make a library popular. It does almost nothing sexy. It's hard to set it up. Feature-for-feature comparisons to more accessible libraries (jQuery+plugins) are unconvincing. It's also verbose and kinda ugly, which folks tend to get hung up on. The various libraries are also very thorough, well-tested, etc., and the build system is amazing -- but that's not stuff that sells.