Probably a bit of all of this except the last I reckon. They probably made them in small batches as weren't sure how popular they would be, add to that people picking the game up cheap now and those wanting to add to extensive collections they hold. I don't think trade in prices of most things are very good these days, unless you have something extra rare (which is almost certainly better off selling on eBay, Gumtree or Facebook), it isn't even worth the journey most of the time. Just look at the starter pack pricing at CeX, they buy the PS4 version for £2 cash or £4 trade, and Xbox One version for £1.30 cash or £2 trade. Unless you live virtually opposite the store, it is hardly worth carting the box and contents half way round the city for that sort of money. Then look at this pack for example, they'll give 20p cash or 40p trade.
I really don't get the pricing with Starlink - The starter packs have bombed (I just ordered the xbox one to go with my switch for the base ship model for £8) but the prices of the ships hasn't really changed but stock is drying up (Argos and Smyths only have about 3 ships models left in stock but not widely available). I was expecting a lot of cheap ships/weapons/characters to be hitting CEX about now so I could pick up some more but they are basically new price once shipping is included. Did they not make that many or are people collecting them now? Or do kids just not trade in now? Or is it still too early?
Coming in at £3.99