The reason why many cryptocurrencies are considered securities is because there's a certain group of people promoting them as in investment. They are claiming that their cryptocurrencies will rise in value.

If you study the definition of money, and what makes a good currency, there's no kind of money that rises in value merely because you own it. Money is meant to be spent or invested, not hoarded. (Adam Smith's "Wealth of Nations" gives a much better explanation in the first few chapters than I can.)

So, the point that you have to register as a security is the point that you start convincing people to own your cryptocurrency as in investment. At that point, your cryptocurrency isn't money; because money is meant to be spent.

How do you keep the SEC out of your cryptocurrency? Invent a cryptocurrency that functions as money. You will need to figure out how to keep the value stable on a day-to-day and year-to-year basis. That means that the supply needs to expand and contract quickly based on demand. "Mining" based cryptocurrencies can not do this.