The most important factor in the growth of gambling is access to it.
Millions had no real access to gambling in 2000 of example, there was simply no way to put a bet on something in most small towns other than buying lottery tickets, and here I'm talking about eastern and even part of Central Europe, not godforsaken places, not even mentioning villages where you didn't have a super store let alone a gambling shop.
Smartphones and internet access changed that.
The second one, despite myths and tin foil hats, is that disposable income grows almost everywhere, people had money, and millions that were penniless now can afford the throw away a few bucks here and there.
Those are the deciding factors, in the end is all about the $, you can't have an increase in revenues in an industry if your players don't have increasing $ available to spend.