We still are "Satanists". We follow Zeus, Satya, who was branded as Satan, and know even the word Satan itself, in this exact form, to be fine and of good spiritual origin.
However, you're probably asking why JoS specifically used Satanism for its name and face, when HPS Maxine already knew even in 2002 that "Satan" was actually Odin/Zeus and that the Demons are Pagan Gods.
This is mostly due to the Enemy, but also due to unseen things I can't speak on or fully comprehend.
On the subject of the Enemy, spiritual groups who aren't built on complete bullshit or scams, and genuinely try to make a name and power for themselves, always end up neutralized by the Enemy, either through corruption of knowledge, infiltration, or slander.
JoS embraced Satanism by looking at the jews' accusations of the Gods and showing that people will still love the Gods and be drawn to them even in their "wounded" forms, as HPHC puts it.
The way the knowledge of JoS was presented was perfectly designed so as there to be as little infiltration by the Enemy as possible, so the matter of the jews' evils, spiritual crimes, etc. had to be made clear. Everything about JoS screamed that the Gods of Humanity were demonized and that none of what is being presented as "the evil side" was truly evil.
There would also be a lot of people born either due to Pluto in Aquarius who would make good Zevists or directly sent by the Gods here to be Zevists. Nobody could really "control" these people, so JoS was made in such a way so as these people were led to it.
I know for sure that, in my case, had ToZ simply been "pagan" and not "Satanic", I would've never ended up joining.
This is not pure coincidence, and all of JoS' infancy was a masterplan by the Gods and a Magnum Opus of HPS Maxine, HPHC, HPS Lydia and everyone else who contributed to the religion's development, growth and survival. The JoS went through ups and downs, challenges and victories, all to cement itself as the house of the Gods on Earth.
Sounds a bit cliche nowadays, but here it truly was "God's plan being greater than man's".