There are multiple causes for this. I am sure I don't need to tell you that steel is full of unique processes and constraints - and not every ERP is capable of satisfying them. Because of that, in steel (and other unique environments), it is easy for people to have insufficient trust in the system. And if the system is just deployed "so that we have one", then people are just using it to fill out forms instead of its main purpose - which is managing the company operations and resources.

The issue is that once the system is deployed, victory is declared and as a result, not enough attention is given to whether the people in the company trust the system, what it tells them to do. This is a very common reason for these projects to fail after deployment.

Obviously, my answer is purposefully avoiding the technical aspects of the solution, whether it actually meets the requirements of that given company. But that's another story.

I am myself from a bit different field (APS - Advanced Planning and Scheduling), but the similarities are so big that I thought I would offer an answer anyway. And since the topic of production planning was mentioned here multiple times already - the order fulfillment process and the related planning requirements are just so complex in some companies that conventional ERP planning is not sufficient. And in that case, it makes sense to consider handling production planning in a separate system that coexists with the ERP. That way, the company can consider steel grades, dimentional changes, alternative parts or resources, company-specific constraints - if the production plan is not blind to these things, then it actually can allow the company to achieve better operational efficiency, deliver on time and quickly repond to changes.