Most likely they do this because those two coins are the ones that have the biggest 24 hour volume so if you have a lot of coins it is by far the safest way to trade even if the profits are not as good, we must remember that the size of your capital is really important in order to decide in which markets you can trade, in some of the most established markets this is not as much of a problem as there is an immense amount of money invested in it and even if the market of bitcoin is still growing up significantly it is still way smaller than those markets.