More crucially, they are winning over executives of medium-to-large internet businesses with superior sales and marketing.
Individual engineers at these places do not have the power to make decisions about which map software to use, which are often six- or seven-figure contracts. Mapbox offers a service that is comparable to Google Maps in terms of price, quality and flexibility. From a technical standpoint, there are pros and cons to both services; Mapbox's toolkit is more modern than Google, which means documentation is a pro for Mapbox but legacy browser support is a con (though it's just a minor annoyance).
Where they far outstrip Google is in sales and customer support. Google Maps won't bat an eye for you, even if you're paying them millions of dollars a year. The Mapbox team, in contrast, will devote developer time to answering your questions, respond to GitHub issues for you in a timely manner, and even create new plugins and repos if you have a legitimate need. That is their big advantage and that is what is driving adoption.
Source: I was employed at a medium-sized internet publisher, and tasked with evaluating Mapbox to replace Google Maps, was a primary developer working on the switch-over, and even wrote a blog post about the experience [0]. They even invited me to their office to work with them for a day (which, regrettably, I wasn't able to follow up on).
[0]: https://blog.rentpathcode.com/webpack-progressive-enhancemen...