It's convention. With browsers, you wouldn't want to introduce a recursion point in TLS (we already have certificate chains, and now we'd get OCSP check chains and where does that terminate?). Apple just did what everyone else does for OCSP, in a way which is accepted practice for good reasons.

Now in this specific instance, OCSP is being used in quite a different use case. For one, the plaintext issue is not a problem when browsing, as attackers can see what sites/certs you're accessing in the clear anyway (certificates are plaintext in TLS sessions), while app launch is an otherwise offline activity. So in this instance it makes sense for Apple to switch to HTTPS (and if they have OCSP on the server cert for that, that should go via HTTP to avoid loops or further issues).

But what Apple did here is just standard practice, it's just that there happen to be good reasons to diverge from the standard here.