These products may be out of stock for a couple of reasons.
First and foremost, they know their main customer, robotics teams, are much less active during the summer than they are during the school year, so it makes sense that they wouldn't plan on carrying much stock over the summer (typically companies want to carry stock for as short a time as possible, as stock sitting on a shelf represents money invested that isn't generating cash flow).
You should also keep in mind that these companies have been known to release product updates in the fall (
example), and they may want to let products that are being updated empty off the shelves before they become obsolete, and to free up room for the new products.
The only way to know when a particular item is going to be restocked is to e-mail or call the company themselves to ask. Answers you may get on CD won't be as accurate as those the company can give you.