Using bitcoin in gambling is a lot more better than using this altcoins on gambling, using altcoin on gambling is hassle because you are going to convert it first to bitcoin, while if you are using bitcoin, there is no need to do so, and the price you might get is bigger compare if you are going to use altcoin. Most of the gambling sites use bitcoin because the majority use it.
Now all gambling sites are allowing players to do gambling with altcoins and bitcoins. The major coin is bitcoin, but they are included few altcoins in their sites. And yes using bitcoin is better than using altcoins because here we have to pay multiple conversion charges and in that time the price varies means without playing gambling we will get the loss. So better if you play with bitcoins no extra charges you need to pay, and it is acceptable in all casino sites.