It is obvious for bitcoin since bitcoin is not a financial asset, but a currency, it can not be categorized as a "security" but when it comes to altcoins the definition of a security is in a way that allows SEC to categorize some of them that resemble "a tradable financial asset" as a security. Although I'd argue that since altcoins are useless and don't represent any real value, even that definition is wrong. But SEC (and the likes of it) are using these categorizations for themselves so that the government has a clear way of how to deal with these things. For example to tax the hell out of them...
But Bitcoin is taxed the same way as any other cryptocurrency in my place. There is no difference at all. When it comes to issuance, like ICOs and so on, there are differences in taxation compared to miners in Bitcoin. But that has different reasons.
Monero shouldn't be a security either then if Bitcoin isn't deemed a security? Or what exactly would be the difference between the two? Even if there are known people who created Monero, that doesn't matter because there are also people today maintaining Bitcoin. Monero wasn't dropped on anyone, but could be CPU mined during its early days, especially when you were familiar with remote access to AWS CPU instances. So nobody actually organized a public sale.
The SEC is trying everything to bend regulation such that they cover as many cryptocurrencies as possible.
You also said that Bitcoin is a currency, but besides El Salvador I am not aware of another country that embraces that definition. How do you get to the conclusion that it is a currency and, hence, cannot be categorized as a security? I might not be all up to date with the SEC's most recent statements.