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.
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.
Also, bitstamp verification time is now 2 weeks, and bittrex has completely paused any new signups.