It's more that the Arduino ecosystem is now so well-supported and wide ranging that it makes sense to have the association with it. e.g., I have an STM32 Discov

It's more that the Arduino ecosystem is now so well-supported and wide ranging that it makes sense to have the association with it. e.g., I have an STM32 Discovery board on the table next to me that has the standard Uno pinout available in case I want to use Arduino shields. For prototyping using the ecosystem is insanely fast. In many cases it also makes sense for low volume production.