Concur with NoSecondBest.
One thing about instructing a class is that as an instructor, you test the communication level between yourself and your students. You can also do hands-on demonstration in class without live ammunition so as to get the students acquainted with their firearm or a variety of firearms before going to the range. This helps build confidence and competence and also ensures that safe handling practices are drilled into the student. As an instructor, you don't want to take any one who is dangerous or careless with a firearm out to the range.
You can't do those things online and the online/distant learning students would be total strangers to the instructor. They will have various levels of competence and safety practices. With the online training, all you're really doing is playing range offcer (or range safety officer).
I can see why the NRA does it though as it generates more revenue directly to the NRA. Second, it makes the training more readily accessible to anyone who can't attend classes because of distance, time committments (work, family, school).