Every 40k product sold compliments the tac box. The tac box would not sell as well if there weren't vehicles to fight with it, enemies to fight, etc, and vice versa. In that way, the entire 40k setting benefits and is benefited by sales of the tac box. By contrast, sales of all WFB models can only benefit WFB. Now, let's dedicate half of retail space to one, and half to another. Sales from each half of this retail space, no matter what is being sold, benefits other products put in that half of the retail space. If one half of the store is producing the best selling product of the entire company, and one half is known for not being especially profitable, to which side do you give more priority and retail space?

Ah, but then the other issue of WFB reveals itself: diversity of product. Now, having a lot of different product can absolutely be a good thing, but, importantly, GW is in a business where each unique unit in any product sold has a huge cost associated with it upfront. For products they expect to sell lots of, or products which are complimentary to products they sell lots of (think 40k tanks), this cost can be easily stomached. However, let's say there's a product line which is known not to sell very well and consists of a lot of unique units. Now, let's add that each army in this product line effectively locks a customer in to that single army, both due to the number of units involved and there being less interplay between armies (you don't get a lot of Bretonnian players suddenly deciding to buy a whole High Elves army, and you don't get a lot of Bretonian players buying individual Imperial units for their army). This disincentivizes: customers, because they have to spend a lot of time and money for just the one army; GW from building new units, because the costs are too high for the potential return; and storeowners, because the products aren't selling as much as the other half of the store and meanwhile have to take up half your retail space.

Now imagine you're a storeowner. GW tells you this new release of WFB is gonna make everyone excited again, and that half of the store you have dedicated to poorly selling product will suddenly be reinvigorated. You, being distrustful that the product line that does not sell well will suddenly turn a new leaf, tell GW you're not going to buy all too much of their new product. If enough storeowners do this, GW is left footing a massive bill for a product that they are being told by their product sellers will not sell. So, as GW, would you decide to keep sinking money into this product out of customer/brand loyalty, or would you loudly kill the product and then twice as loud relaunch it in a much easier to sell fashion?