It was a neat surprise to get the back-to-the-future feeling, but why the "welcome to stackoverflow"?
This is actually addressed in the post on Meta.SE announcing the joke. The idea I believe is in deference to the fact that Stack Overflow was the first site. Purely conjecture, but it might also have to do with the fact that it was simply easier to make one universal banner that could be used for all of the sites.
Here is a quote from the relevant section of the above mentioned post:
What's that? It says "WELCOME TO STACKOVERFLOW" you say? I asked our designers about that and they tell me dank fonts don't need lower case. Oh, you mean why isn't it STACKEXCHANGE? That's a bit of a time travel paradox. We set the baseline at 2008, but the Stack Exchange brand didn't really show up until a couple of years later. We have our crack quantum mechanics working on it. Once they patch the timeline, we won't have any way of knowing.