Preorders generate risk for a business, because you take money and owe a customer goods, potentially for months. You cannot spend that money because you need to be able to refund it, but you're also carrying the risk of storing and protecting that money because it's technically yours.
If the date, costs and shipping of the next batch aren't even known yet, it makes no sense to open up for preorders. Also, the company risks taking on more preorders than stock they can get their hands on, which will only help to disappoint customers who will have to wait more weeks for the next batch to arrive. There's a lot that can go wrong for the company.
If it took more than a few days to sell out for each batch, it might make sense because it'd allow for the store to be sure that their entire volume is moved. From what I've seen, these watches go out of stock in hours. There's no need for the store to carry risks to make ordering extra convenient for its customers because they will sell out regardless.
There's benefits for batching orders before you back order from the factory, but there's also downsides. From a business standpoint, I don't see the advantage of taking on back orders for a product that sells out instantly anyway.
If you're anxious to by the Pinetime, the hardware is practically the same as the Colmi P8 if I recall correctly, and the open source OS can be flashed on that with some extra steps you can find online. You can find that watch, and many similar ones, on your favourite Chinese import website. You lose the official support, but you'd get your hands on a hackable watch without setting a calender reminder.