I'm sure Casey could answer more accurately, but I'm also sure he's busy running a business, so while I have a few few minutes free I'll take a crack at it.
Many (if not all) of the ABDL diaper suppliers do not have their own factories, as this would cost millions of dollars to start up and would be very unlikely to pay back your investment, so instead they contract with other companies that do. More often than not these companies are based in China, or another offshore location, so orders need to be a relatively large run each time, primarily due to the fact that it costs the factories more money to set the machines up to manufacture diapers than the actual cost of raw materials (needing to be amortized over the amount of diapers they do in a run), and also to minimize shipping costs across the run as well.
You as a business owner then need to factor in how much of what to buy, what is going to sell, and what you can fit in your warehouse. You need to be able to predict how many people are going to want each product in between orders, and if you guess over then you're stuck with stock taking up space (and money) that could be used for a better-selling product, if you guess under then you miss out on potential sales - which is where we have the SDKs out of stock "all the time". Demand has outstripped supply, not good because you don't have product to sell, but not bad because you have room for growth.
But why not just order more? Well, with normal shelf products you could do that and have it in short order. With a specialty product like ABDL diapers that are made to a custom specification, you have to place an order, get scheduled for the machinery (it's generally in use 24/7 with various orders from around the world), have your production run done, approve it, have it loaded on a boat, wait a month for it to show up at port, wait for it to clear customs, THEN be delivered to your warehouse and stocked. There are so many things that can go wrong in that chain (this is likely why the DC Amor 2018 has delay after delay), and Chinese factories are well known for not being punctual, or even sometimes not being forthcoming about potential defects.
All you can do is wait, and be sure that everyone (buyers AND sellers) wish you had your SDKs just as much...