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 the point is, it's still gambling that played the biggest role in why Bitcoin took off. Sure, exchanges were there because you needed to convert fiat to Bitcoin to gamble, but without a reason to spend it, like using it on gambling platforms people wouldn’t have bothered exchanging their fiat in the first place. There were only a few users before, but it slowly grew until the numbers exploded.