Hashut’s Blessing:
Tarrak, the price of some extra pigment isn’t worth passing on to a consumer usually, particularly if there are two colours being sold (considering it would reduce some sales for one of the products).
Also, you DO have to add extra pigment to turn it from grey to black. Plastic is often made in white/cream (as GW’s used to be), so add a little black pigment (like with the paints) and it becomes grey. It’s exactly like adding food colouring 
You’d need to add more black pigment to make it more black.
With the other models (which I have bought before), they usually have a purpose for being a different colour (tanks are often painted green or are army toys and green is the colour thought of by children for the army, cars are more often than not already in the final colours etc etc).
Ultimately, we’re just considering the real reasons, but it may be best to ask GW themselves, lol 