There is a time cost to learning a new system. And a lot of people don't want to pay that cost to wean themselves off a proprietary software.
(A lot of "poor UI" complaints are really "UI is not identical to what I trained on").
But that has to be weighed against the risk!
At any moment, a proprietary single-supplier can choke you right out of business, by suddenly deciding to charge (or charge more) for access.
That's my main business reason for FOSS tools.
@TerryHancock in the broader thread there are plenty of examples of specific functionality that is missing from FLOSS alternatives. It's not just "I don't like that the button layout is different".
@doctormo @nicemicro