One of the reason is a legacy systems. Some companies are too tied up to old custom made systems built on old software and hardware virtually not convertible to cloud. You will be surprised but there are big corporation still using AS400 and not planning to switch anytime soon. If you heard in recent news US unemployment system was still built on COBOL... In 2020...
Another reason is a cost. I love AWS! Its fantastic to be able to create and launch servers or the whole farm of servers in the matter of minutes! And ability to convert physical servers to virtual and upload them to the cloud is breathtaking! But my monthly bill started at $300 and grew to $18K per month in less than 3 years. And that was for just a few virtual servers with Windows OS and SQL. My company realized that we can have a state of the art datacenter with WmWare and SAN on premises for the fraction of that price. Put second one on the other coast (one of our other offices) and you have your own cloud with better ping and six digits figure saving a year.
For the last I would name vendor lock. With vSphere its very easy to move your virtual servers between AWS, Azure and Google (assuming you can afford all 3 and licensing cost of WmWare) but have you ever tried to "download" your server back to premise? It's virtually impossible or made so hard by cloud players trying to keep you up they're in the clouds.
With all said I read that Netflix (I believe its Netflix) saving hundreds of millions dollars per year by using Amazon instead of its own servers. I also read somewhere that Dropbox moved away from AWS...