Because it's the fastest way to transact big amount of money with a low fee. And for gamblers, it's a big deal for them being anonymous especially to the countries that prohibit gambling. But, it's hard to convince people that are not a big fan of using tech.
Gambling i risky thing which wastes our efforts with our greediness, I think people using it because of the popularity of bitcoin, I do believe it is too hard to use bitcoin is a currency in many countries. But it would be easy to use bitcoin as an investment for a better future in long term.
I strongly believe it is time to use bitcoin as an investment.