Grant Petersen's very popular book "Just Ride" basically describes the difference between what we have come to know as a "cyclist" and someone who simply rides a bike. Differences being mostly about bicycle race marketing cliches and riding on'e bike in a more comfortable appreciative mode of identifying one's own ride whatever that might mean to you.
"Ride your own ride" I have enjoyed saying from time to time, knowing there are no absolutes and there are exceptions to every rule. If you have spent any time at all commuting where bike lanes are not prevalent or bicycle commuting just hasn't had its place in the area you live then you probably realized at some point that some rules are made to be broken anyway. That may contribute to the popularity of converting a mountain bike for bike commuting. Converting can be as simple as replacing heavy knobby tires with tires which provide less rolling resistance or something in between. Other additional changes to a mountain bike might be replacing the stem or handlebars to provide a more comfortable riding position or adding fenders and or racks.
Kindred Cargo Bikes for Bicycle Travel
Whereas a traditional touring bike is based on a fairly standard idea of frame geometry for comfort for extended time on the saddle, a commuter bike is mostly defined by the rider's commuting needs depending mostly upon road conditions and cargo needs. Some bike commuter folks may not find it necessary to install a front rack for panniers, while some find it convenient for their bike camping or other touring bike needs.