One of the problems with people who make a lot of these standardized control panels is they want to be able to substitute parts from different manufacturers based on what is the cheapest part available right this second. This is not a big deal for current limiting fuses, since UL considers a Class J, CC, etc fuse made by anyone as equivalent, but it can make a difference with other parts. For instance some contactors have been series tested with Class CC fuses, while other with Class J fuses, and some with both. But if you want to use whatever is cheapest right now you have to accommodate all situations, and it is just a lot simpler to use the 5kA rating so you don't have to figure out all the permutations..