I would say it is very popular because it is easy and does not require any change in lifestyle or belief and yet it is very effective. Other powerful means of meditation require taking vows of vegetarianism or other strict behavioural vows but with TM there is no such requirement. Also the organizations are very much an open book and extremely transparent. Both the fundamental teaching organization the Maharishi Foundation USA and the largest charity organization the David Lynch Foundation have to and do post all their detailed financial information online and anyone can see exactly where the money comes from and goes ( mostly to providing scholarships to those who cannot afford to learn and a decent standard of living to the full time TM teachers especially those who teach in prisons, inner city schools and just cities with very expensive costs of living.
According to the renowned Psychiatrist Dr Norman Rosenthal TM never does anyone harm and it benefits everyone who is willing to do it, and he states it is the only meditation that has proven physical benefits. With TM you have a lifetime of full-up in the form of the checking procedure to ensure correct effortless practice which is usually free of charge and I have never, ever witnessed any form of “up selling” involved with checking(or ever for that matter) as someone has claimed here. Also with TM you know what you are getting, it is taught in a very standardized way to ensure that no matter where you are you are taught by a completely competent qualified individual. The only reason so many people are out there recommending it, celebrities or otherwise is because they are so deeply satisfied with their own results.