Different specific locales have at least some of the data, just hard to search down.
I'm not surprised at the difficulty to dig out good high-level data. Knowledge is $. The equipment manufacturers groups and dive societies all want you to pay $ to access more current/complete data. There are some people who do professional seminars on the dive industry -- again, pay $ and you'll get to see the statistics on diving and expected growth.
The societies get their info from people joining them, the equipment vendors get their data from all those little equipment cards that they ask you to fill in when you do warranty registration. Govt. agencies generally don't have the interest/$ to take dive-specific censuses, although again there are local initiatives/studies out there, just not worldwide information (short of getting the societies to part with their closely-held member data).
A variant of the PADI slogan -- "Buy things -- fill out cards -- be counted!"