why is there this special site for Ubuntu?
I think that most of the questions are general Linux questions and would belong there
Well, as an active and relatively high-rep user of both sites, I can tell you that the style is very different. While the blog post explaining their differences is out of date (newbie questions are very welcome on U&L, the "for advanced users" is no longer part of the site description), the main points still stand.
Ubuntu users are very often new immigrants from the Windows world. They use the GUI for everything, are scared of the command line and have little knowledge and less interest about the internals of how a computer works. They basically just want the darn thing to work and forget about it. Of course, this is not true of all Ubuntu users, it is just a user type that is over-represented in the Ubuntu community as opposed to, for example, Arch.
Don't get me wrong, there is absolutely nothing wrong with that attitude, there's no reason for everyone to be a geek and it is perfectly valid to want to use a computer without really knowing how it works under the hood.
So, the AU community has a lot of questions about using the GUI and everyday operating system use and configuration. By contrast, the U&L community has a lot of technical questions asking for in depth explanations of how things work and the nitty-gritty details of the issue at hand. While both types of question are on topic on both sites, the type of answer you will receive is very different.
Then, Canonical have made all sorts of changes to the stock kernel and have even come up with their own desktop environment, Unity, which has not been particularly well received outside the Ubuntu world. Questions about Unity are unlikely to be answered on U&L because very, very few users of U&L would be using Unity. Unity is not off topic on U&L it's just not of any interest to most of its user base as you can see by the dearth of questions and few answers on the tag.
Also, very few users of U&L use Ubuntu. Therefore the community won't know how to answer Ubuntu-specific questions because we don't tend to use it as our operating system and won't know about its specific quirks.
All these points mean that the Ubuntu user community is very different from "traditional" Linux ones. Not better, or worse, just different and it makes sense to have a separate community for them. Also, there are far more Ubuntu users than any other 3 distributions combined (with the exception of Linux Mint but that's very very similar to Ubuntu anyway).
Finally, the scope of the two sites is very different. On U&L we accept questions about all Unices, AIX, OSX, BSDs, any and all Linux distributions (including Ubuntu), embedded systems etc. AU on the other hand has a very narrow scope and only accepts questions about Ubuntu, Edubuntu, Ubuntu GNOME, Kubuntu, Ubuntu Kylin, Lubuntu, Mythbuntu, Ubuntu Studio, Xubuntu and Ubuntu Touch. Anything else is off topic. Therefore, the two sites are needed since they each cater to a different audience and have a separate, though overlapping, scope.