I have never understood gambling on mobile phone, because it is such a small screen.
Optimization! Gambling sites optimize their sites in mobiles in such a manner that you won't really feel inconvenienced by the small size. It's way easier to start gambling on mobiles when compared to gambling on laptops or desktops.
It really doesn't matter how much you optimize because I have big hands and phone screens are quite small, which means no matter how amazingly you optimize it, and how responsive you think it is, the screen size of a phone itself is not big enough to justify me playing there. iPads are a bit better, they are bigger in size at least and I can understand that but I never used it, well I used it before at my friends and all, but never owned one because I never needed it so far.
However, if we are talking about PC? Well that is just perfect, huge screen, total control, everything on screen. It's just unfair comparison and my choice is clear in this regard.