they didn't give it the chance. Making a proprietary box like that needs real support or the users will abandon it. That is what happened. People wouldn't have minded so much if they could have swapped in their own components.
They were selling mostly MacBook Airs and Pros by that time and the Late 2013 iMac was challenging the 2013 Mac Pro in certain use cases. They thought they had it covered and doubled down on the iMac (Retina 5K) instead as the better bet for most of their user base, even Pros. They were partially right. Mobile was always the better (and frankly, easier) bet from a revenue perspective. Yes, they dropped the ball on Pros and we’re way too slow in course correcting. The Mac Pro may or may not be the answer. Time will tell.