I'll take a stab as an armchair linguist: Hieroglyphs still represent written language, in the sense that each glyph represents an abstract sound, and glyphs can be composed to form words. Emoji do not represent language, in the sense that they do not have a vocalization, and emoji do not combine to form words.

Take, for example, the various skin colors for faces and persons: if emoji were a real ideographic script, the written representation would be a logograph combined with a determinative, not a set of distinct glyphs. The irony of course is that is exactly how it is encoded within Unicode (an emoji codepoint with a skin color modifier). But doing it this way is exactly why emoji is an illegitimate script: it does not represent any non-digital form of writing, and the emoji modifiers do not have any representation of themselves, neither visual nor audible. Nor is the modifier composable in the way that a real language would be: it does not modify animal colors, for example.

Because they are one color. Like a font, their defining property is shape.