I have saw a video in-depth with same question. Candy Crush popular because of they've published for smartphone based operating systems. Apart from OS they've composed a devil music which addicted to play more and complete next level.
So i can give you list of points why you so addicted to candy crush.
2. Smartphone Operating System Game
4. More levels
5. Delicious stuffs
6. 1 - 20 levels are very easy so gamer would think above 20 levels would be very easy.
7. Facebook (Facebook audience has already got Candy Crush's attention because of it were one of Facebook Games List)