Well, it also took Apple quite a while to finally have a classical music app optimised for the iPad, so it may well just be as you said. It probably doesn't take that much effort to get it done, and the developers at Apple may also have a hundred other things to scratch off their to-do list, each also maybe taking a couple of hours to resolve, so it's really a case of competing priorities.
That said, I just created a new journal entry about my morning run, and the app was able to suggest a snapshot of the route I took, together with the distance and time (indicating it is able to tap onto other sources of system data like fitness). Data which the iPad doesn't have access to (the fitness app on my iPad just has a popup stating that fitness+ isn't available in my region), so another possible explanation is that there isn't an iPad journal app because the iPad doesn't have access to the full range of data that the phone has.
But it's probably just a matter of time. It would be nice to be able to look back at my journal entries on a larger display one day.