The other reply to you is wrong. The answer is because a chain whose direction is controlled by a single entity in practice is probably a security as its value

The other reply to you is wrong. The answer is because a chain whose direction is controlled by a single entity in practice is probably a security as its value depends on the performance of that entity. Bitcoin's value as an asset is not contingent on some other party with control of it, unlike say Ethereum, Binance Coin, etc. Dogecoin is arguably in the same category as Bitcoin. So forks don't have to be securities. They do have to be actually decentralized, as in one entity does not decide which fork is the true fork (like with Ethereum, Bitcoin Satoshi Vision, etc).