Is there a reason big Bitcoin companies wouldn't use only SegWit transactions?
I would like to give one of the reasons (Disclaimer: I am associated with Blockonomics). 

There is no support of segwit change addresses yet in bitcoin core. There are workarounds that need to be done to achieve this; as was done by Yodice in this thread. Also to create segwit receiving address one needs to use addwitnessaddress. Lot of businesses just directly use the core wallet without doing lot of programming (there is fear of what may go wrong).

What is needed is that bitcoin core wallet works by default segwit without lot of tinkering. I hear 0.15.1 is better. Looking forward to that