One of the things I have learned over my decades doing this gun business is that, in the world of manufacturing, gun stuff is small business. Items (guns, parts, etc.) are generally manufactured in small 'runs' and are unavailable between runs to everyone, even Midway. The timings of these manufacturing runs is made by the estimation of sufficient need and often made far in advance (months). They guess wrong sometimes. Other priorities/contracts may over ride a scheduled run too. A couple examples I can easily remember.
1. I need some 32-20 brass. Midway didn't have any and no one else did. I finally called Starline brass and they told me they had a run scheduled in about a month and a half. Sure enough, they sent me some directly and all of a sudden Midway and everyone had them. Hard to fault the retailers.
2. My friend wanted a left hand Remington 870. No one had any. He backordered the gun in January with the knowledge the next run was to be made in August (a few years ago). Hard to fault the retailers.
3. I want/need some Speer Gold Dot 100 gain .327 bullets. Everyone has them backordered. I called Speer months ago for the date of a production run. They gave me one. That date has long passed and no one has them yet. The production run may or may not have occurred. If it did, the supply they produced may have gone into the production needs of their own .327 ammo. Who knows. Hard to fault the retailers.
I have experienced zillions of similar examples. These manufactures are relatively small and make only a few of their catalogued items at any one time. A bullet or brass maker may catalogue scores and scores of different bullets/brass, but make only a couple dozen types in a given month. Some types may only be made every couple of years. There are few production lines in constant production for any one item. Some might be for items like .22 and .223 ammo, items where the national consumtion is huge.
In theory, a retailer like Midway should be able to play this game and estimate how much of a given item to order to last to the next production run by estimating demand. They probably do a fair job at this, but demand is fluid due to a variety of factors such as those generated by politics or even whims generated by magazine articles/tv shows on shooting. The retailers are also at the mercy of the inner workings/needs/priorities of the producers. These needs may change monthly and sometimes on a whim.
BTW, I am very happy with Midway. They are among the best in the business. Sometimes the best places to find these hard to find items is small shops that experience little demand. This is where the internet can really be your friend. The bigger places (Midway, Grafs, etc.) may be long out and waiting for a production run, while some some small internet retailer may have some the the back shelf be sheer luck. So may some mom and pop gun shop.