I suspect the design choices were also driven by the desire to streetpass the notes to people over a matter of seconds, which probably led them to design the data to be ultracompact and not really that flexible.
You're right Vahne, these are design choices, not technical limitations (except maybe in the case of any streetpass hard data caps? Do those exist?). In this case, early design choices must've restricted their ability to add new features down the line, like colored ink.