That being said, this is FANTASTIC. Due to the nature of lua it may well always lack a good community but thankfully people seem to be trying.
As far as penlight goes - feel free to open an issue or PR to discuss any inclusions/exclusions. I haven't actually used it myself, but as a fan of microlight I decided to include the big brother. Everything can be reconsidered.
Is that for technical reasons or because Lua is commonly used in proprietary apps?
The user base is fragmented (version 4, 5.1 (LuaJIT) vs 5.2), as Lua is usually used as embedded language (alongside C, C++, ...) for video games (World of Warcraft, Far Cry, ...), applications (Adobe Lightroom, ...), etc.
And the http://lua.org website is a bit too minimalistic to offer community features. The project only hosts a mailing list and a old-school wiki. The additional libraries are/were hosted on http://luaforge.net , an outdated sourceforge clone. The documentation is static and the development process itself is not so open. The Lua language standard library is lacking functions almost every other language ship with their default package (so everyone has to either find & download additional libraries or reinvent the wheel).
Other languages like PHP have a documentation that allows the community to add comments and code samples.
Penlight is the closest thing there is to a Lua standard library (loosely modeled after what exists in Python). It is maintained by Steve Donovan who is one of the most active members of the community and the author of http://www.luafaq.org/, LDoc and lake.
Having contributed to Penlight myself I can say it is certainly not poorly maintained compared to most other Lua projects.
I agree, Penlight is not the best at everything it does, and it probably does too much (which is why there is microlight), but it is still a tool most Lua users need to know about.