A great book tangential to this subject matter is Jaron Lanier's "you are not a gadget" which spends a substantial amount of time on the development of music in parallel to computers. My recollection is that midi basically became the dominant format because it was first, and it was first because it was more or less the most obvious way to represent music in a simple programmable format. Other formats that allow for more flexibility didn't crack through due to the critical mass of adoption of midi, so it persisted until today.