Because it's very convenient format that, even with free tools, can easily be converted to standard notation for those who want or prefer it as well as to midi for those who prefer to learn by ear. Some even choose abc purely for the sheet music it can produce.

Now tell me Dick. How are you going to do all that with your piece of sheet music. Would it be a hand written sprawl that could only be photocopied or rewritten or would you using notation software to do that. And if the latter, in doing so, would you be completely oblivious that the software would be converting some (in most cases unreadable) other representation,internal or on file, format into your dots~?