One thing we always consider is the fact that we'll have to support the thing forever. So we have to make the call on if the thing seems like something worth supporting forever.
We should be clear though: there are LOTS of things you can use on CodePen that don't require us to specifically allow it. Anything that is front-end only will work fine. You can link up any JavaScript file you want. You can link up any CSS file you want. You don't need special permission for that, and that covers a huge amount of possibilities, and more all the time, as browsers grow more and more powerful.
This can be tough to communicate. One UI struggle we're facing right now is our little "Quick Add" dropdown menu. It's got popular JavaScript libraries in it, and was originally an attempt to make adding them to Pens quicker. But it works against us. Too many people think those are the ONLY libraries we support. Actually, you can just start typing into one of those External Assets inputs and it will autofill from 1000's of libraries for you.