Hardware

It's not technically hard. The tech is quite well understood, and I want to say is kind of basic when compared to other fields.

What's hard is getting certifications for all the different telecom regulators, getting interoperation to work, and signing deals with others to access their patent licenses without paying super onerous duties.

If you recall, that's Apple's weakest point. Apple doesn't like to work with others (see Nvidia, Intel, etc) - the computer industry is filled with Apple's dumped ex-partners and telecom modem is the one place where you need cooperation from everyone. Apple needs to suck it up and basically be nice to other companies, and that's the one thing Apple doesn't do.

When apple does decide to play nice with others, that's when you'll see the modem release.