Reason for Lowercase usernames

I can see validate_user() calls sanitize_user() with strict argument as true, which means uppercase letters are not allowed too.

In source code, I found this comment

// If strict, reduce to ASCII for max portability.
    if ( $strict )
        $username = preg_replace( '|[^a-z0-9 _.\-@]|i', '', $username );

I would like to know how does this affect portability?

Uppercase characters are not blocked in usernames on single site setups.

Uppercase characters ARE blocked in usernames on multisite setups. The wpmu_validate_user_signup function forces lowercase a-z and numbers 0-9 only.