The biggest factor (IMO) is that during the pandemic chip shortages, about the only WiFi/BT module in stock anywhere was espressif. They consistently had stock when no one else did, so a lot of home hackers got into it.

As our industry is so fond of forgetting and rediscovering, if your product is easy and accessible to home hackers, it will be used in those hackers' real jobs.

But also they're so goddamn cheap. Espressif somehow scaled production super fast and got the price way below any of the competition.