Because there are so many thing you will not find in Bitcoin. Entertainment, fun, music, communication with other gambler, adrenaline, enjoying to play game etc it's can be find in gambling. While Bitcoin is just a currency and you're either invest or trade it, nothing more. You can't play with Bitcoin because it's not something that will entertain you.

But the good thing about Bitcoin is, if you're a techy you might be interested to learn the technical stuff which Average Joe not find it as fun.