And when you're talking about camera systems, the camera itself (which is where Sony currently has the lead in mirrorless) is not the biggest factor in choosing a brand.

Canon and Nikon both have tons of specialist lenses that are still not available (or only have one flavor specializing in one space, at one very high price point) on newer mirrorless platforms.

And if you've invested $50k-100k in lenses (e.g. for a photography department) are you going to turn on a dime and dump that gear for new fancy Sony equipment that costs more than your old glass? Likely not.

And many PJs still need some of the qualities that you can really only get in top-end Canon or Nikon DSLRs (as someone else said, ability to drive nails with the camera, and huge grip).

As a concrete example, imagine if you were tasked with taking footage or stills of one of the protests at night. Would you rather bring a rock-hard large-gripped body that could even be used for protection? Or a smaller body with a screen that would break from a minor scuffle and cheaper plastic, less waterproof construction?

As long as the flange distance of the sensor from which you're adapting is significantly more than that of your camera and you manage to decode the autofocus communications protocol, you can adapt them.

Short version: optical physics, proprietary link for autofocus, capability differences

Really aside from Nikon, most AF protocols are fairly well understood and can be adapted to other lenses.

(in the case of Nikon, sometimes they go out of their way to break others using their systems/protocols without a license...)

But you can use larger mounts on a smaller mount (with care!) May not need a bigger tube, but perhaps a longer or shorter one.