I think they just associated Poison with the colour Purple ya know? Which is why they changed Poison-type Pokemon to Purply Psychic Types in the TCG.
Ice is linked to Water, i.e. Ice melts and turns into the liquid H2O, so Ice should stay as Water-types in the TCG.
Iunno, I've always saw the TCG as having 8 types (when Metal & Dark types came out way back when, I was totally hyped up about it!), so I guess I've always took it for granted. 8 types is enough though. After all, MTG only has FOUR types, and they're working out pretty well in game mechanics.