>>3693974135 has been the industry standard for a century, it has the widest selection of lenses and the most support behind it.
FF has twice the sensor size of crop, which means twice as good signal to noise ratio (the objective measure of quality of any signal), and the size of the format has almost zero effect on the size of the camera as they need to be a certain size to fit comfortably in your hands - digital mf is comparatively huge due to knowing that mf mounts will need to at least support 645 in the future, making it unwieldy for a lot of professional use. The larger sensor of ff over crop also has a large improvement to resolution and sharpness as digital camera resolution is limited by the lens, not the sensor.
Digital mf is by no means comparable to film medium format - which whilst more standard was still a niche. Digital medium format doesn't come close to the smallest film medium format for sensor size, 645, which was seen as a consumer wannabe format in its day with professionals sticking with 66 or 67; it's barely 50% larger than ff, the jump in quality from crop to FF is twice as large.
And finally price, getting started with digital mf is $10k+, whilst full frame can only set you back around $1200 even buying new, and to get a crop camera with comparable build quality is going to cost around $800. So you're doubling your performance for 400 bucks, as opposed to marginally improving it at the expense of unwieldiness for $9k.
Once mf at least hits 645 sensor size, and costs no more than double that of a similar full frame, then they may be a viable option to be taken seriously.
Most pros that do use Digi mf use digital backs and only in studio, it's not a great user experience but the image quality is good, but you're looking at $50k+ to get started.
The "but why not use mf instead of ff" people here are just buttthurt crop users with no money.
