The dice sites can take any cryptocurrency and convert them to tokens for playing and again convert them back to currency when withdrawing. This way a site can support many coins without causing any lag on the game.
I don't think it matter about PrimeDice's coding abilities, they are perfectly capable of creating whatever is necessary to accept altcoins. I think it's more of not being necessary to PrimeDice, as altcoins can become a pain sometimes, with their fluctuating prices and how many there are.
If prices of altcoins are fluctuating and that is a problem then bitcoin is king of stability then. With price spikes of more than $300 in one day.
It is not about fluctuation, as PD or any other casino don't care about this. It is more a waste of time and effort to introduce something new for less than 1% of your userbase.