Because dice in my opinion is the easiest game to play. As well as this game has long played. Thus, gamblers are familiar with gambling types of dice.
Even if your not familiar with this game also you can start playing within a couple of minutes because it is so easy to play and that's why it has become so popular game for online casinos. I also started online gambling with dice game apart from cricket betting.
People tend to play dice because its very easy to either win or lose. There is no special skill one has to master before they can start playing and no need to research like soccer games.