What's so bad about fish for cats anyway?

I thought that fish & milk are cat's favorite foods I know that's not true, that milk can upset a cat's digestive system, but why isn't fish nutritionally sound for kitties?

The high magnesium in fish is thought to contribute to the formation of struvite crystals.

What are struvite crystals? Also will it be alright to feed food with fish every once in awhile?

There is some information on struvite crystals here. I know many people who feed fish-based diets and their cats never develop crystals. On the other hand, I think I have feed *maybe* 2 cans of fish based cat food (over her 3 years) to my female and she does have struvite crystals. I don't know how much of an affect fish has, but that is why fish is supposedly bad.

Thank you