The site probably didn't realize that there would be a lot of interest from people who like to play in the currency of real money. The GBP is very stable. The Euro is slightly less stable, but in any case it does not make a rollercoaster like the Bitcoin. And that roller coaster of the bitcoin can rise rapidly but also fall hard. Then you have more security with a Euro. You will be disappointed if you play with Euros and see that the price has increased.
Any of the known currencies will work just fine, whether it's GBP, Euro or USD, since their volatility is nothing comparing to crypto as they don't change more than 1-3% even in the most eventful day.
The important part is that a gambling website should give their users the option to use stable coins as well even if it's just USDT it's still better than not having any form of stable coins available, crypto market is volatile and it's always better to have the ability to use a stable currency to do your gambling and get your profits if you want to.