1) Front-running. It used to be rife on the mainstream stock exchanges until they made it illegal. 

Front-running is where you know what your client will do next and you trade before him, knowing that his later trade will change the price in your favour.

The exchanges know how much cash their clients hold.  They also have power over how long it takes before they credit you with the dollars you just sent to the exchange.  They say it takes one to eight days.  It should be the same value day.  They can credit all clients on a Sunday night, knowing the wave of buying will start on Monday morning.

Once your coins are on the way to the exchange or already arrived its not worth the time or trouble to move them to another exchange.

2) For exactly the same reason as above, most exchanges don't take Tether.  If they did, you could instantly arbitrage between exchanges, thus undermining their profits from front-running.