Can't you normalize these fees? Bitcoin fee 1.7% like the others would be amazing. I have been using other services for bitcoin for this reason.
This is not mining fee difference
There is no reason for such difference. Bitcoin mining fees are much lower than ethereum's for example.
It would seem quite strange that there is a higher fee for Bitcoin transactions. Bitcoin with the highest volume and the lowest spread. The only explanation would be that exchanges do not want to favour BTC swaps, because Bitcoin suffers the most from checks and AML analysis, while other currencies do not so much.
it would certainly be more useful if a CCE.cash official gave a direct explanation instead of sharing tweets or other statistics.