Derivatives like Mint and Backtrack are not the same as Ubuntu. They use different packages, from different software sources and typically built differently. They are configured differently. Their principles and goals overlap with but are not the same as the principles and goals of the Ubuntu project. They have their own communities.
Many of the same problems that can happen on Ubuntu can happen on Mint, Backtrack, Debian, Fedora, OpenSuSE, and a variety of others. Some are relevant to Windows. But we limit ourselves here to answering questions that are about Ubuntu as we define it--Ubuntu and its official derivatives (which share package repositories, development philosophy, and substantial elements of design philosophy with Ubuntu).
There are excellent support communities for these other operating systems--communities specific to them, like the Mint forums, and also more general, like Unix.SE.
The main thing we're better at is--apparently--looking good. People sometimes come to us even when their questions are not about Ubuntu, because they've heard we're good. We should take that as a compliment. But we're not doing them or our community any good by trying to answer questions that are not about Ubuntu. A significant portion of the time, we would assume something in the derivative is similar to Ubuntu, and give bad advice.
There's often no way to know at the outset if a question's answers will be Ubuntu-specific, Mint-specific, or otherwise. We send people somewhere they may get better help before failing to help them instead of after. In situations where we would have succeeded, other, more suitable resources would likely also succeed.
There is another reason that is just about making the site better and not (or not directly) about doing what's best for people who come here with questions about Mint and other unofficial Ubuntu derivatives. As the about page says:
Ask Ubuntu is a question and answer site for Ubuntu users and developers. It's built and run by you as part of the Stack
Exchange network of Q&A sites. With your help, we're working
together to build a library of detailed answers to every question
Anything not about Ubuntu is clutter that gets in the way of maintaining Ask Ubuntu as a resource for information about Ubuntu.
Finally, I believe there is a reason pertaining to the well-being of the Mint community (and other such communities). Suppose we allowed Mint questions and some Mint users embraced Ask Ubuntu as an excellent place for them. Then the Ubuntu community would come to be seen as an authoritative source for information about Mint. Mint (and most other unofficial derivatives) are deliberately unofficial; most don't want to be subject to the will of the Ubuntu community or the governance structures in it.
So it would be bad if we were to absorb Mint. It would take power away from that community. Some people use Linux Mint because they want to be part of the community and governance structures set up for it and not for Ubuntu. We should respect people who choose to partake in other communities.
Efforts for greater cooperation are a good thing, and I think we do need more of them. But unless the overwhelming majority of Mint users want to be assimilated into our community (either partially, for support purposes, or all the way, by becoming an official derivative), I don't think it's a good idea to have Mint questions here.
In contrast, support communities outside a project that are not themselves officially affiliated with and governed by another project are a great way for users of different operating systems to help each other. Unix.SE is one such community; Super User is another. (Server Fault is another but its scope is narrower, see their FAQ.)
I recommend taking a look at the meta question where the community consensus about unofficial Ubuntu derivatives happened:
You also asked the related question:
Is it because Ask Ubuntu is funded by Canonical or anything like that?
Good question. No, we're not funded by Canonical Ltd. Ask Ubuntu is part of the Stack Exchange network--Stack Exchange maintains and administers the servers that make Ask Ubuntu happen. They have a substantial degree of influence over the site.
Canonical does have an impact though; for the last several Ubuntu releases, Ask Ubuntu has been an official source of support for Ubuntu and is explicitly recommended as a place to get help by the slideshow displayed during installation. Searching help.ubuntu.com even includes search results from Ask Ubuntu.
If you want to know more about the relationship between Canonical and Ask Ubuntu, between the Ubuntu project and Ask Ubuntu, the degree of control Canonical or the Ubuntu project has over Ask Ubuntu, and how Ask Ubuntu fits into the "civics" of the Ubuntu community generally, I recommend posting a new meta question.
(For questions about Ubuntu community or governance that are not specifically about Ask Ubuntu, you can ask on main.)