It appears that Microsoft decided against adding Cortana to Dynamics 365, but the Redmond, WA company gave no definitive reason for change. ZDNet found Microsoft's decision on dropping Cortana on the Dynamics 365 Team blog.
As noted in the Microsoft blog post, there is no explanation for dropping Cortana integration from Dynamics 365:
"After carefully consideration, we've decided to discontinue the Cortana integration preview feature. Dynamics 365 will no longer appear in Cortana’s notebook, and Cortana will no longer prompt users with relevant information about sales activities, accounts, opportunities, and meetings.
We welcome your feedback and ideas."
Microsoft has no given a particular reason why it dropped Cortana from its Dynamics 365 platform, but so this is strange news given that Microsoft originally included Cortana integration as part of the Dynamics 365 roadmap released earlier this year.
Microsoft has not yet made an official statement on its decision, but OnMSFT will keep you updated.