Money.

It's very surprising to me that Mr. Oppenheimer would gloss over this one reason with as lame an excuse as, "Well, other religions tithe." True, other religions ask for monetary support. I can't think of one that doesn't need it. They may even, like Mormonism, put a lot of pressure on their members to provide support. But no religion actually requires it.

If you'll permit some light/half-baked philosophy, here is the reason they don't require it: any quest for supernatural salvation cannot rely on the natural. In other words, the Creator is not going to require your money. He or She or It invented all thoughts and material things in the world, including money. So how could the Creator possibly want what they already have an unlimited supply of? To get out of the world you are not going to be able to rely on anything in the world.

So, while Scientology may or may not be a cult, it certainly is not a religion, or at least a religion that can provide supernatural salvation.