Why do the Japanese often use katakana even for Japanese words? Second attempt

New theory, brought about by communicating with my wife back in Japan mainly with SMS for a month:

Because there are so few variations in what you can say (social language is very fixed and for some reason every food stuff must have “oishii” written on its packaging, as if that is going to be enough to convince you…) any variation at all, e.g. in script, comes as a welcome change.

If you have any comments or other ideas, ヨロシクね