I have access to industrial use only products at work, but am no expert on the OP's question. I would imagine that there are many reasons, but most revolve around consumer,product, and mfg. liability. Sometimes the only difference between the two products is the actual printed labeling on the container. In other instances, the industrial product requires a more trained person to use it correctly for various reasons: safety, applicability/success, application equipment, etc.. At a hardware store, the consumer only has to decide which color of Rustoleum they want. With an industrial product, they may have to decide on a dozen or more different primer, topcoat,and additive choices and a dozen different ways to apply it. Consumer products tend to be dumbed down to meet the needs of the audience at hand. Consumers often don't recognize the value of a product that is initially much more expensive.
Again, there are many reasons. And, "industrial use only" does not always guarantee a better product. Contractor grade materials are often marginal products offered to professionals that need something just "good enough" for the situation at hand. The term "idiot proof" comes to mind when I compare consumer vs. industrial grade products.