With all these new chains popping up why aren't people just minting NFTs onto bitcoin directly?
To do this then you need to add smart contracts to bitcoin, and that will not happen.
The good news is you can make smart contracts with other coins like Cardano, Stellar, Tron, EOS... so, the NFT's aren't just for ETH. But since the platforms to sell the NFT's use ETH, then is the most common way to make them.