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.
But some people still gamble using bitcoin, so both are tied together, bitcoin is a tool while gambling is a place to play so playing games can use bitcoin, there are no statistics that gamblers are more fans than bitcoin, if you look at it from your own point of view it may be true that there are so many gambling fans but apart from that there are also more bitcoin and crypto fans around the world.
There are no statistics that show why gambling is much more popular than bitcoin, even though the two are different I think both of them need each other because gambling requires money on the internet to be used like bitcoin as well as bitcoin can be used in gambling machines, so both of them need each other