Great question. This is simple economics: there wasn't that much of a demand so there really was no incentive to make them fresh daily. You might have been the only customer, but that isn't much of a profit-incentive for the company. However, you could also argue that they would be losing a customer if they don't have that daily doughnut (who knows? Maybe you'd buy a coffee and other doughnuts if they would've had that doughnut).
Yes, it could very well be that I am the only customer (actually, it's a franchise, so they're sold in hundreds or thousands of branches, so I would perhaps be one of only a few). Here's another thought: what it has nothing to do with the number of customers or demand for lemon-filled donuts? One of the things I notice is that many of their donuts are a mix and match of a variety of ingredients or things like glazings, sprinkles, flavors, sugar coatings, icings, strawberry jelly, etc. Many of them are a mix and match of these ingredients and things. The lemon filling, on the other hand, seems to be rather unique, or maybe they only use it for only one variation of their donuts (i.e., the one that I like). What if the reason they discontinued it is because they figure it's just one donut, and they could save a lot of money by no longer ordering the filling just for that one donut, they no longer need the extra space or section for either prepping or displaying those donuts?
In response to your last part, yes, they did lose a customer for another item they sell (at least last weekend when I stopped there), which is that I also get a medium ice mocha latte; that time I left the shop without purchasing that too. Not only did they lose me as a customer for ordering that latte last weekend, but they may have lost me as a customer the way I used to be on a regular basis. Now I might only go maybe once a month or every other month to get that latte.
I'm going to try again this weekend to see if they're selling these lemon filled donuts (and the latte); if they don't have the lemon filled donuts again, I'm not going to order the latte either & I'm again going to take my business elsewhere. I think Henry Rogue has the best response so far; sometimes companies make bad decisions.
Did you know that originally MTV used to play music videos (at least they did constantly)? I used to watch MTV when they did. Now I don't even understand what they're showing or who their audience is, but basically I don't watch MTV anymore & haven't watched it for years because of all that. I'm wondering if this donut trade franchise is headed down the same road. They've gotten into offering more and more non-donut products, which I have nothing against; but I wonder if they're just discontinuing probably not just the lemon-filled donut, but others along the same lines, to make more shelf or table space in their existing stores for other non-donut products or equipment for making the other products.