“Games that use blockchain technology or let users exchange NFTs or cryptocurrencies won’t be allowed on Steam, according to a rule added to Valve’s "What you shouldn’t publish on Steam" list.”
Valve’s stated reason is that items can’t have real-world value. This is the company that runs the CS:GO skin market. The real reason is that Valve doesn’t want a cut of someone else’s scam when it can run its own marketplace. Epic says it’s open to NFT games, which tells you how badly it wants Steam’s leftovers.