Your question can be answered in three points:
1) Personal preference
Some people prefer to shoot with a DSLR, possibly because they grew up shooting film SLRs or even the early DSLRs. Some may also be under the impression that DSLRs are just plain better than mirrorless because that's what seems to be the dogma that the online photography community generals passes around as "fact". There are some photographers that like using the A6000, and now the A6300, because they are such small cameras with such high burst rates. Some however, prefer cameras with large grips over small size, and thus go for DSLRs.
2) System requirements
People that shoot sports and fast paced action tend to be very demanding on their system. It is generally well known that the large DSLR systems cater better to these needs. For example, those huge telephoto lenses that sports photographers often need are only available for DSLRs. Canon and Nikon both have their share of 300, 400, 500 and 600mm lenses. The longest Sony offers for Full Frame mirrorless is their recent 70-300 zoom. You could stick that on an A6300 and get an equivalent FoV of 450mm at the long end, but that still does not match up to a 600mm lens, which on an APS-C body produces a whopping 900mm (Nikon) or 960mm (Canon) equiv. FoV. DSLRs tend to be more responsive as well. With an OVF, you can see action as it happens, whereas with EVF, they generally have a tiny bit of lag and as such the image you see through the viewfinder is more of an "afterimage" than a true view of what is happening. As such sports and action photographers generally opt for OVFs which can only be had with DSLRs. There can also be the point that DSLRs have better weather sealing than mirrorless, but there doesn't seem to be any inherent limiting factor of mirrorless that impedes the implementation of weather sealing, but rather just at present mirrorless cameras do not have weather sealing as good as a DSLR (though that could very well change in the future). In addition, Sony is only just putting out "professional support", such as providing loaner equipment and fast tracked servicing of gear. This is something that "pro" DSLR manufacturers like CaNikon have offered for years, and so the customer base that buys CaNikon are likely to stick to CaNikon for the love of their great pro support. My last point for this section, which may be controversial, is battery life. With a mirrorless camera, even though the viewfinder, you're essentially using a form of Live View, and as such the sensor is always active and always chewing through battery. The battery life will suffer. Action photographers cannot risk having to change batteries in the middle of an event and miss a critical shot.
3) Public perception
A controversial point, but it is generally accepted that a large DSLR screams pro over a small mirrorless that "looks like a toy". It can be argued back and forth that mirrorless cameras are used by pros nowadays to create some stunning work, but at the end of the day, a pro needs to use gear that makes them look like a pro, and unfortunately the public perceives a large DSLR like a D5 or 1DX to be the epitome of "professional" (which for some literally is so). There are stories online of how photographers have been turned down work due to their use of "toy-like" mirrorless cameras, simply because the public does not understand that mirrorless can produce great images.
4) In defence of Sony
So this last point is to say that despite Sony A7r II generally getting a bad rep for action and sports photography, it can in fact be used for such purposes. However, as someone who does not shoot sports or action, I prefer to think of the A7r II as a high end piece of precision equipment meant for slow-and-mid-paced work rather than dealing with fast situations on the fly. Pro DSLRs are designed to take a beating, since action demands fast response, and that can mean getting careless with one's gear. The A7r II has a good AF system, but it still generally does not compared to DSLR. Instead, the other good features of the camera, such as its 42MP sensor and broad dynamic range make it great for landscape shooting and fine art. So to that end, I'd like to say that even though you can use an A7r II for sports, why would you? There are camera systems out there designed for the task in mind that does the task better (though better is subjective), and though the A7r II can be used for action with great amounts of patience and skill (being able to anticipate the action to a science), it is generally done better with a DSLR.
Hope this helps!
Cheers!
