Ask HN: Why prohibit spaces in usernames?

My preferred user name has a space in it. In the early days of the Web, when you tried to create an account somewhere and it wouldn't let you use spaces, it meant that their user management system was so rickety that it thought spaces were field separators, and had to be protected from itself. By the mid-2000s, most sites had sorted themselves out and allowed spaces.

In the past few years, this seems to have reversed itself. More and more major, professional sites--Hacker News included--are again prohibiting a character found in pretty much every human name. Some sites don't even have an explanatory error message--you try to create an account, and it says "Your username is invalid" and leaves it to you to figure out why. Fiverr doesn't even do that, it just put a red border around the text entry field and glares at you.

Is there some solid technical reason for this, or is it so kind of giant contagious mental block? The only thing I can think of is wanting to parse usernames out of posts Twitter-style ("@PhasmaFelis"), which I've seen in a few forums, but most sites with this issue have no such feature.