Enhanced performance, designed for simplicity – the new Outlook for Mac

Because thats actually four different applications across three different systems. Its not trivial, and to have the amount of features (its not just email, its shared notes, project management, calendaring and contacts as well) and it quickly becomes a larger project. And without a guaranteed payoff it makes it harder to justify the amount of development time. It has to be clear-cut better than both the free builtin applications (which work fine for a lot of people) and better than old Outlook (which is decent for the next level of users), and good enough that people are willing to pay at least $60/year on top of their O365 subscriptions to fund continued development to handle the constant work needed to keep working with Microsoft's ongoing changes (and Google's, and Apple's, and still with IMAP and POP). It's not an easy business case to make. 

I've gotten by with Mac Mail, BusyCal and Busy Contacts, and Reminders and Notes and it is functional (I do love the Busy apps as an upgrade to Apple's built-in software), but yeah, doing it all in one application is way easier. I still stick with old Outlook, because even with its limitations the integration is just cleaner.

It does feel like the dev push has seriously fallen-the progress seems to be slowing dramatically, and some basic table-stakes features still remain to be implemented.