Not only due to the adoption in gambling, but also due to the evidence it gained over time, with payment gateways, influencers who started talking more about it, the popularization of exchanges sponsoring events, there are many actors on this stage, not just gamblers, many things contributed to this happening.
But at that time (2012), it's like it was only gambling platforms that recorded the highest transaction for Bitcoin because not much business were interested to add Bitcoin as a payment options and many people were still so skeptical about Bitcoin while so many people saw it as a scam. If it's now, you can see how so many institutions are not adopting Bitcoin on like before when they showed no interest. Regardless of how some institutions have adopted Bitcoin now, there are too many crypto casinos that is accepting Bitcoin for gambling.