Our previous host, Wikia, listed the following two types of licenses as not allowed:
NonCommercial: This used to be easy to see the reason for. Our previous host, Wikia, had ads along the right side of the page. Now this isn't quite as clearcut, but using a NonCommercial license makes your work incompatible with other projects that don't accept NonCommercial licenses.
NonDerivative: When your work is resized and placed along with other work in the wiki, it could be thought of as a derivative work after that. It's better to therefore avoid the ambiguity and avoid work with NonDerivative licenses.
Then we get into the next type of license that I've made a decision not to support in my art drive:
Permission to use on WikiFur: There's no problem with hosting them, but I feel they should be avoided. All the text on WikiFur is licensed under the GFDL and CC-BY-SA. Anyone can download a database dump and host the text on their own servers. If we have to switch hosts again because they go out of business or implements policies disagreeable to the furry community, all our work is saved. In our first host switch from Wikia to Timduru, it was clear which of the two was still WikiFur. In the future, it may not be clear after a switch whether the project is still WikiFur, and a WikiFur-only image might not be able to make the jump. I won't have my art drive go towards art that's tied to one company. Also, releasing your image under a non-restrictive license makes it available to the TurriWiki (Finnish furry wiki) and other spinoff projects.