You are right as long as the betting amount is fixed in the value of $ then it wont be a matter, but the most used reason why people prefer for a coin to gamble is about its popularity, its price, and the speed of transaction.
The amount when you bet could be fixed but its value may decreased because the price of the coin is dumped significantly. In this case, people prefer to use coins that has less possibility of getting dumped and high possibility of getting pumped.
Bitcoin is still the most popular and the most used coin in this crypto gambling industry, the other coins is just alternative when they have no bitcoin on their wallet or they want to use bitcoin for other purpose such as holding or trading.