I am on SO a bit, and about ten times a day I see questions that are purely DBA questions. I flag them every time, but it doesn't seem to be getting better.
What is causing this
Continue to flag them, that's what that mechanism is for. Additionally, leave comments on the question referring to
[dba.se] (that's the comment shorthand to provide a full URL to this site properly decorated) and asking if they've seen this site, so that they can be made aware of this site.
One reason they are posted on [so] is because that's what people know. They know that they can ask those sorts of questions on Stack Overflow and get answers pretty quickly.
and what can we do, as a DBA community, to promote this site so DBA-type questions end up here?
You can do just that, promote this site.
However, I want to go back and ask you to clarify what Jeff Atwood already asked you to clarify in the comments above. How do you know they are "purely dba questions"? Is it because the questions are about database configuration? Is it because they are about how to write a query? Is it because they are about how to design a table/schema?
The reason I ask is because, while I don't consider myself a dba during the day (I have two absolutely smashing dba's that know stuff I won't even pretend I know, and they can tune things to do phenomenal things) I do exactly those things you're referring to above.
I design my initial schemas (because I know the source data) and I ask my fellow coders if the schema makes sense according to the design/scope of what I'm doing, and I take it to the dba's after and make sure it makes sense.
I install the software on occasion, so I run into config errors too. Errors is errors, anybody can usually help you sort those out. Runtime configuration errors made by a user (programmers are users too) doing something wrong may need expert help, and for those, yes, I may ask my dba's.
Writing queries is our forte as programmers, that's what we do best. If you need help writing a query then you probably need help writing code (since queries ARE code) so you should ask those questions on Stack Overflow. But when you need help optimizing your queries, this is the place to come to get immediate feedback.
So here's my little rule-of-thumb: If I would have to go and interrupt my dba's at the office to ask a question of them, it probably is fair game here. If it's something I could figure out without stopping their entire workflow (dba's are programmers too, they can't suffer a 15 minute interruption without hours lost either), then I will do my best to figure it out on my own.
Do you agree or do you think that this site should be about every thing that might even only involve databases a little bit?
Because I'll remind you that iOS applications also store everything in a custom database structure on the device filesystem, so that would mean this site would be wide open to all iOS problems as well. I think you can see that "anything even remotely related to databases" is the way madness lies.
So instead, we should apply my little rule of thumb. If you have to go and ask your dba's everytime you write a query, or everytime you design a table, or everytime you install some software ... then yes, ask everything here. Otherwise, let's leave coding on Stack Overflow, architecture on Programmers and app software issues on Super User