I can only assume that there is some population of speculators that are only able to purchase bitcoin cash through Coinbase.. maybe the other exchanges make it too difficult?
A number of friends of mine in the US only want to deal with Coinbase. It is probably a combination of having already set up there and not wanting to submit ID to other exchanges, and having trust issues with other ones.
Also, bitstamp verification time is now 2 weeks, and bittrex has completely paused any new signups.
In the US, if you link a bank account to Coinbase then you don't even have to submit an ID. So the barrier to entry is significantly lowered compared to other exchanges.
No, can't really be the case that they can't rather they would just have to be too ignorant to do it elsewhere, if you can buy Bitcoin it's relatively trivial to buy any other crypto currency.