It's not an efficient use of the time slot / effort that goes into it.
40k events, whereas it might not seem like it, take a LOT of time to prepare for. Ard Boys takes considerably less time for various reasons, but it still requires time, energy, and though.
It's just big enough where it turns the regulars away for a Saturday, which costs the store money. It alienates a large part of the community. It has no entrance fee. It's supposed to "drive sales" while people bulk up to 2500 points, but it doesn't. It's run at 190238982390842093849024 places on the same weekend (prelims). It doesn't allow me to do what I'd like to do. It gets in the way of doing something that is better for the community, players, and the store.
Basically, instead of BG running Ard Boys, I can run two more 40k events every year. The store wins in a huge way, something like 3-4x more players get to play, and it gives me another great reason to ask for more tables / terrain.