Why is Candy Crush still so popular?

I never stop seeing people (of a certain demographic) playing Candy Crush on public transport, and I continue to see adverts for it all the time. I'm in my 20s and it must have been about 10 years since I last played it and can't remember another viral mobile game being relevant for more than a couple of months - no one talks about Angry Birds, Flappy Bird, 2048, Doodle Jump, Temple Run etc anymore, however many new installments of these games the developers might release, I guess because the novelty wears off quickly and they become boring. Can somebody who still plays Candy Crush please tell me what is so engrossing about it after all these years?