I'd say there are three main reasons: 1) Legacy systems not supporting special characters, 2) The desire to keep support overhead down, and use of the system u

I'd say there are three main reasons: 1) Legacy systems not supporting special characters, 2) The desire to keep support overhead down, and use of the system up, by keeping users' passwords actually rememberable. In other words, if you let users make and use paswords they can't remember, they will, and as a result they'll either annoy you about it or stop using the site due to the hassle, and 3) Developers not wanting to have to worry about parsing special characters from untrusted users, which could potentially be dangerous. In the past the top reason was probably the lack of ability to support the passwords (#1), and today it's probably the lack of desire to support them--mostly from #2, but partially from #3.