Because of Binance domination in cryptocurrency industry, their competitors like centralized exchanges, have policies against Binance and Binance Smart Chain BSC. They don't want to automatically give free advertisement and listing for tokens on BSC, or listing fees with BSC tokens can be more expensive than ERC20 tokens or other blockchains.
I of course can not say about reasons why Roobet are against BSC listings but they have reasons for it, and fortunately nowadays there are many chains, not only BSC, so it's not big problem for users.
A bettor can choose other cryptocurrency to make deposits which their fees are far way more cheap compared to BSC, and of course you are right for what you are saying because many of these centralized exchange doesn't find it pleasing giving free promotion to Binance Smart Chain, and some gambling site and exchanges doesn't allow and/accept deposit from BSC networks rather than other more chain. Just as user above mentioned, Doge, LTC, Trc(usdt), XRP, SOL and POL. All these network are also cheaper while using them to make deposit and withdrawal from some of the gambling site that listed these coin to their sites.