Cost. Especially if you have a large amount of users, and you already have the IT staff and servers to run it on (you can try to justify it with less admin costs and less server costs but the numbers don't add up for some orgs)

As an example, I worked at a healthcare org and it would have been $70,000 a year for hosted Outlook, but only $110,000 to buy the software outright and run it for ten years (700,000 - 110,000 is a big savings)