Sometimes the reason they can't keep it in stock is because there are supply problems, such as when demand greatly outstrips the supplier's ability to supply requested amounts. If this goes on for an extended time, it can become a choice between dedicating shelf space to an item which you know you're not going to be getting sufficient stock, and just dropping that item in order to free up shelf space for a competing brand you know you'll receive.

Sometimes the chain's warehouse doesn't ship enough to the store. The item is ordered by the store from the distribution center, and yet corporate makes an executive decision that a certain store just doesn't need to carry that particular item any more, or doesn't need as much as was ordered. That particular store may sell a lot of it, but if another item sells even more at that store, they dedicate more shipping space and more shelf space to the bigger seller.

There's also a push in the grocery business to get customers to switch over to store brands as much as possible, since the profit margin is a lot higher on store brands than on national brands. Many times the store brand is made by a national brand. That doesn't mean it's made by your favorite national brand, or one that is LC friendly, only that it's a duplicate of a national brand.

I don't know if it's this way in other stores/chains, but there are certain often requested items that our store can't order - the district warehouse just sends more when it suits them. For items that the individual store has the ability to order, sometimes the ones who do the ordering at the store/district/corporate level also just don't order enough for various reasons, including glitches in computerized ordering systems.

A lot of manufacturers pay big money for prime shelf space in grocery stores. The more the manufacturer pays for shelf space, the more shelf space there will be dedicated to it, which squeezes out some of the competition, even competition from well known national brands.

You'd think it would be controlled almost exclusively by customer demand, but it really isn't. The bigger the grocery chain, the less control the individual store has over what and how much they stock.