Has Linux on the desktop really fallen so far in the last years? When did the decay really set in?
How does open source friendly Microsoft not see Linux email clients support falling behind at least as a problem in MS documenting its APIs, if not an opportunity to participate in the major open source clients and influence and attract developers?
Even if the exchange protocol was open, I wouldn't implement support for a specific vendor knowing there are protocols which are more commonly used.
Microsoft could instead start by improving their implementation of IMAP in exchange, which is one of the most horrid and buggy implementations of IMAP I've ever seen. This is also true for their IMAP endpoint at office365.com.
But it's clear where they stand: outlook is the ultimate corporate lock-in, and it's not due to email by itself.
IMAP and the raw message/rfc822 format is already a quite complex format to handle. Most open-source software starts by building their logic around these formats.
Exchange is a completely different model, everything from how you communicate with the server, how you store the bits on disk or in memory, or even the logical structure of messages. On top of this, it adds several features that don't really have a parallel in the standard RFCs. For most people working on email clients, it is far easier to tell users "enable IMAP on your Exchange server" than to actually implement the full top-to-bottom stack you need to get things working. Even then, given the likelihood of bugs and missing features, it's hard to justify making the switch from Exchange to $ALTERNATIVE, and if you didn't need those missing features, well go back to enabling IMAP and use that instead.
The APIs that MS uses for Exchange are largely public at this point. Even the mapping between MAPI messages and message/rfc822 that Outlook does is released in one of the documents.
It's currently a toss-up whether it's even worth trying to use Outlook on Windows desktops, as much of a buggy, hogging, piece of garbage it is.
I suspect it doesn't bother them hugely that desktop client support is falling behind on any platform.
edited to add: Oddly I can't find much marketing by Microsoft of the browser Outlook, otherwise I'd link it here, but I'd say it's about 85-90% feature complete compared with the desktop client.