I did some superficial digging and APPARENTLY Kellogg’s has trademarked “Kellogg’s Frosted Flakes” and they’ve trademarked the specific logo design but not the term “Frosted Flakes” itself.
My bet is it’s because the trademark examiner ruled that “Frosted Flakes” is too generic/descriptive to be a trademark (it’d be like trying to trademark “Grapes” to sell your grapes)