The general argument is that iOS is more secure because Apple strictly controls both the hardware and the software. The two are very tightly integrated in Apple products, which means proprietary software for e.g. segregating userland or interacting with the secure enclave is specialized and well-audited.

The other (perhaps more compelling) argument is that Apple is only able to invest this level of security in its products because of its spectacular profits, which would likely be much lower if iOS were open source.

That's basically the gist of it.

EDIT: To whoeever downvoted this, I'm just relaying the argument.