Why can't you install OSQA on a webserver just like Joomla/Wordpress?

This is the common fallacy of Open Source software: there are people out there who both have lots of free time to develop code (obviously they are independently wealthy and don't need a paycheck) and they like to give that code away for free. If you read a lot about the Open Source community you will find neither of those things are true.

Wordpress is a great piece of software, which has been in existence for about 8 years now. With the huge user base (it is considered one of the top blogging platforms in the world) it has attracted a number of developers to work on it and improve the code. It is the popularity of the platform that puts it in the "unique" category of open source - it has a large developer base and can maintain a release schedule. But this is the exception, and not the rule. Most Open Source software is (if maintained at all) done by a very small group of people who have to prioritize their time developing features.

The group that heads up the OSQA development is more the normal Open Source platform development group. It has a few developer who have done a great job, but expecting them to have a completely "ready to use", "easy to install" package when it hasn't even hit release 1.0 yet seems a little bit unreasonable.

If you want a complete working, out of the box, solution for Q&A platforms I suggest you try the QATO platform. It was written by the same people who are writing OSQA.