Bitcoin is fully supported. However, the way Coinpayments works is that when the fees are high, and the amount "too low", they hide the coins with high fees. Current average fee for a Bitcoin transaction as of writing is $1.13. This would be doubled as there's a conversation involved. Meaning about 23% of your $10 transaction would become fees.
Note that with cryptocurrency you also pay a fee to send the payment, this feature is to protect both buyers and sellers from losing most of the payment to fees.
For the same reason Bitcoin over Lightning network is shown, as well as over alternative chains like Binance Smart Chain where the fees are lower. There is currently also a 5% discount when paying over the lightning network, offered by 99Stack Cloud.