Item colors show more information than ASCII colors, though I agree they don't look as good for it. Their color follows a spectrum based purely on rating, because you can already differentiate subtype by the tile design/shape. By comparison, with fewer glyphs ASCII must use color to identify subtype, so the two systems are not identical.
Using different colors for different items makes the map even harder to parse in tiles mode because there will be too many differently-colored large objects. It's easier to get away with in ASCII because punctuation is generally smaller (pixel-wise) than letters.
Certainly I'd prefer better colors for the tiles mode, but most of the other colors have already been used (including some you guys haven't seen yet), so there aren't a lot of options for that. I could make an option that colors all item tiles the exact same color if anyone might want that, but it would lose the extra rating hint.