There are even a lot of other OPEN SOURCE projects with licenses (Apache, BSD) which cannot use readline because of their restricted GPL (instead of LGPL).
Well, this isn't correct, it's just that the result would have to be distributed under terms of GPL.
(Assuming the license is GPL compatible, which most of the ones you mentioned are).
This project goal is a bit more about replacing readline, but a big motivation for the original project is minimalism and trivial embeddability. It's all in the README.
http://thrysoee.dk/editline/
Edit: libedit is mentioned in the readme, and it's implied that the reason for this other library is code size.