The reasoning behind it being that our services are based around sustainability and ethics (including data privacy). When we run the server we know where it is hosted, that the data server is energy efficient and powered by renewable energy. Similarly, we know that user data isn't going anywhere else and isn't being tracked by anything.
But running mail servers is a lot of hassle. Not so much the actual server or software stack, which is all great, but maintaining reputation and deliverability, and support.
Oh, and it helps if you love acronyms: SPF, DKIM, DMARC, MTA-STS, BIMI, et al.