"If the network on which the token or coin is to function is sufficiently decentralized – where purchasers would no longer reasonably expect a person or group to carry out essential managerial or entrepreneurial efforts – the assets may not represent an investment contract. Moreover, when the efforts of the third party are no longer a key factor for determining the enterprise’s success, material information asymmetries recede. As a network becomes truly decentralized, the ability to identify an issuer or promoter to make the requisite disclosures becomes difficult, and less meaningful.

And so, when I look at Bitcoin today, I do not see a central third party whose efforts are a key determining factor in the enterprise."

https://www.sec.gov/news/speech/speech-hinman-061418

They are fully decentralized networks that no one controls. A security requires some issuing company or person.

Almost every other cryptocurrency has one or a small group of person that can take control of the chain / app.

Securities do have to register using form S-1. This is required for IPOs, for example.

BTC is leaderless, which seems to be a large part of why the SEC is convinced it is not a security.

ETH is not leaderless to the same degree, and to my knowledge has not been declared "not a security" by the SEC in the same way BTC has.