I don't know why but Hikari seems to do a better job of making their food appealing to the fish than many other brands. I read the labels and see that there are definite benefits to other brands such as Northfin in terms of ingredients and nutrition...but although the fish eat both types they seem much more enthusiastic about the Hikari products.
It's probably analogous to kids preferring McDonald's burgers to other better foods. Just because they like the stuff certainly doesn't mean it's good for them. I know that if I am weaning a predatory fish off of live food I seem to have an easier time of it using Hikari Massivore pellets as opposed to Northfin.
I think it's the density of the pellets. When I fed Hikari pellets in the past every fish would take to it but when I switched over to pellets there were more nutritious I also noticed how they were much more dense. Likewise, when I would attempt to feed the new pellets initially the fish still had vigor but when they bit into the pellets they quickly spit it out. Only exception I can think of was my Umbee, he was a tough mad lad who ate up anything and everything.