...Both projects have their own coins: STX (Stacks) and SOV (Sovryn).
We need something without other coins. The reason? If these coins / this network don't exist anymore ... so your NFTs won't. But Bitcoin will exist ... always.
STX is "mined" by spending BTC, and the source code for doing so is available freely. So, one might argue that as long as BTC exists, anyone who cares can also make STX exist.