Let's say Airfix make a production run of 10,000* of a partictular kit, and various retailers order 9,950. The 50 left-overs still need selling, so they are made available on line direct. Airfix will give preference the the retailers because their network gives the biggest exposure to Airfix products. They are a vital tool (whether they're a bricks and mortar or on line shop) in the business model. Airfix have to make a judgement on how many sales a particular kit will realise. The injection moulders won't just knock out a few kits, a viable production run will be thousands, so under-estimating demand will neccessitate a further production run while over-estimation will result in left over stock to clear. In the ideal world Airfix will make exactly enough for demand and those kits will be available from your preferred stockist, but it just doesn't work like that. Sometimes we have to look harder for what we want.
*All figures are ficticious and purely to illustrate the point.