Scientologists say it’s not actually a cross because it’s 8-pointed (there’s 8 “dynamics” in Dianetics. Long story, and it’s all bullshit anyhow.)
Anybody can see it’s intentionally made to look like the christian cross. The real reason is probably that it’s a cross because Hubbard figured it’d make people easier to scam – a cross looks nice and “safe”. There’s also some pretty funny woo-woo about how Christianity is just an “implant” put in us by good old Xenu, and I think the cross shape supposedly somehow triggers our “thetans” (scientology’s equivalent to souls.) And yes I know weird amounts of details about scientology, I’ve always found it fascinating because it’s such a bonkers belief system