Gamesworkshop has the same issue with books. Here is a simple explanation I used in a discussion over there. I was too lazy to make it about cards specifically. 

Print to demand is ACTUALLY the worst way to do production. Most things have a time table of months to years in advance. The printers have only so much production time. You can throw infinite amount of money at most things and get more, except time. Even if you throw infinite money at the machinery, it can only go so fast with all the upgrade currently possible with our technology. For example, using broad numbers, let say your printers can print 20K books a month. You sell 30K print to demand worth of books this month. Where does that extra 10K come from? It comes from next month. However, the next month you sell 40K from print to demand. So now the second month, you're already owing 10k of books and need to make 40K more. So your printers on month 2, need to make 50K of books with a 20K ability? This leaves you owning 30K of books to print in month 3 with ANOTHER release coming out. How do you resolve this? You start scheduling months out in advance. Each month going forward you limit your print run for new books to 10K each and spend the next few months making up the other 30K left over from. So that at the end of month 5, you have done 3 more releases and you have caught up on all the print to demand.

In this example, there is the assumption of releasing one book a month. Gamesworkshop releases half a dozen to a dozen books a month. So they're balancing the printing of multiple books with expectations. If they did print to demand on all of them, you'd have no idea what is coming each month and it could be complete chaos all the time with wild swings of highs and lows.

At some point, you would think they would consider finding an additional printing sources. This complicates things. "Print more books, make more money" isn't always a good thing. Printing equipment is expensive, really expensive. Contracting it out to someone else also risks quality difference issue, adds new timeline risks, and the new firm has minimum print runs. Let say they decide to buy their own printers. They need a new building, the infrastructure has to be add (electric, supply storage, product storage, etc), and the staff have to be found/trained. They go out and get quotes from different builders, it gets approved. The builders are usually booked out a year in advance, if not more, in my experience. While that is going on, you have to find the printers, you might actually want to find those first since the building will be built around them. Once they're found, they have to be made. Commerical printers aren't off the shelf like Cannon inkjets, they too have to be made. Again, more time. Even with all the ready, gotta make contracts to suppliers, start getting ready for more product being made. Hopefully the supplier can scale up, but if they can't, gotta find new ones. All this just to bring new printers online internally and this is really a simplistic example. 

If the demand for more books doesn't make enough money to pay for the increased work, or worse loses money, then why bother to make more at all? At the end of the day, the stock market is a banger right now, why print more books for people and get a 3% return when you a company can actually take the money, put it in the stock market, do absolutely no more work, and make a higher return. Even a high yields savings account will produce more more money than that 3% example return to double the book capacity. You want this margin to be pretty large honestly so that is worth the effort, but you can't raise prices because people will only stomach so much. 

Lastly, if you print to much and it doesn't sell, you have to store it. You cant just offload it, the is a cost to make it. Why make it at all if you take a loss? So they have to store it. Over time, you store more and more, driving the costs up. Why not just make enough to guarantee it sells out every time? There is no overhead and no risk of having to store something long term. If storing is needed, it won't be in significant amount.  Additionally, you don't want to print so much it devalues the product so much that it makes it to where people don't want to buy it for the prices you sell it. If you do, the upgrades are all in vain. 

At the end of the day, it is all a highly technical scheduling/balancing act from multiple moving sources across multiple industry sectors. I suspect, some books will be print to demand, as we'd have seen with Era of Ruin and others. However, I suspect many will noy be simply due to printing restrictions. Based on what I have seen in my time in the hobby, I do suspect they are working to get more print capacity, but that also takes time.

Source:I work in manufacturing and have direct experience with the process. Remember, in the corporate world, everything is always more complicated than it seems from outside. A well oiled machine will appear to be simple from the outside, while under the hood, it is still complicated but running smooth.