That the first 32 characters are control character has absolutely no relevance, because you don't have to use them to get 128 characters. We have 256 characters to choose from, and only the first 32 are control characters. That leaves 192 characters, and therefore 128 is completely possible without using control characters.
Here is the reason: It has to be something that will look the same, and that you can copy and paste, no matter where. Therefor it has to be characters that will be displayed the same on any forum, chat, email and so on. That means that we can't use characters, that a forum/chat/email clients may typically use for formatting or disregard. It also has to be characters that are the same, regardless of font, language and regional settings.
That is the reason!