I I would say because it's really to easy to play and gamble with BTC, and dice is the most popular bitcoin gambling game in the world. People want fast and easy money and dice is fast and easy, that's it, maybe.
This question all relates to why do people gamble? I think everyone wants to earn ("free") money, right? Some of us are willing to pay for a chance to get more money, thus some of us are willing to gambling. Others are not, although they buy lottery tickets etcetera. And others are not willing to pay something (for a chance to win money).
And why people play dice in specific, I think because dice games are well known and quite easy to understand. (Online poker takes more knowledge for example). And, many dice websites can proove they're legit and fair. The reason why people use BTC for this gambling is because it's really easy, isn't it? I can't imagine a faster way gamble online than with using BTC for this. It works great on websites like Primedice.