I am very fond of Tanzania (my brother and father were both born there) and East African diving generally. But the main reason it is not more popular is not hard to discern - it is hard to get to. From North America it is the far end of the earth. From Europe you have to go past the Red Sea first, which is pretty good, and even if you don't stop there, you then have Sudan which is also excellent. And from Asia? Well, lots of world class diving nearer to home.
Sucks for you, but I am not unhappy that Zanzibar hasn't become a mass tourism destination yet.
Sucks for you, but I am not unhappy that Zanzibar hasn't become a mass tourism destination yet.