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...