I think if you want to learn physical skills purely to defend yourself, it should take as little time as possible.
A lot of martial arts seem to say "It'll take 5 years to get to a good level". The bottom line is with work, study, family etc.... people just don't have a lot of time to train. People in days gone by seemed to have nothing
to do but train. Times have changed.
A lot of people have found that martial arts ask an awful lot but offer little in return. Here is a great blog about that very point:http://dysfunctionalparrot.com/martialarts/quit-karate/
I've certainly heard that opinion more than a few times. So I can see that if a course offers results in a quarter of the time of traditional martial arts, that can be appealing.
Additionally, as Cord pointed out a while back, public perception of TMA has shifted over this last generation. TMA are seen as more as a joke rather than a serious system of self-defense. Look at Ross in Friends or Dwight in the Office (US). That could put people off too. People see stuff like Krav Maga, online programs or even MMA as offering better value for money.