I think it's because, unlike almost every other cosmetic anything in the game, these framer's kits aren't meant to look good, or have options to fit whatever theme you're going for. They're purely designed to show off how high a rank you got in a given season, as such there's no need to hand out the lower rank kits to a player in a higher rank. Otherwise, why would the season number be included at all? Why not just make them season-generic and let players collect them?
Is there anyone who likes that kind of design? Anyone who's chomping at the bit to show off that they got Crystal in Season 1? Anyone who cares?