I would think for the sets to sell out quickly is what Topps wants to see.
You might be right, although with Old School, they were clear that they wanted product available in the store for some time.
With the postcards, perhaps they simply want them to create buzz within the hobby market. And, whether it's trading cards or video game consoles, shortages create a lot of buzz. Maybe that's why an unlimited set was not done this time - to get people really excited about Topps product.
Of course, there is a big potential downside to shortages, and that's what we've also seen a good deal of, at least here on the forum. So, as much as people are excited, perhaps even more people are upset that they missed out.
Back in the 1990's, Playmates Toys had a highly successful enterprise with their Star Trek action figures. The sales were good and they were quite popular with fans and collectors. The company decided to do special "1701 editions"; three different figures in a year, each produced in extremely limited quantities - only 1,701 of each. What followed were tens of thousands of collectors who had, up until then, been trying to get them all. Suddenly, those buyers/collectors were off the hook. And they stopped buying. The line's rapid decline seemed to follow directly from that marketing stunt.
No doubt, those 1,701 figures sold out the minute people found them (they were randomly inserted in case packs) - but it did not serve the company, or the hobby that surrounded the figures.