The first reason is money. Those who can afford to buy a king-sized bed can afford to buy the best and most comfortable. They do not need to buy an additional mattress topper. On the other hand, couples who can only afford to buy a double bed are not normally the type to splurge on memory foam toppers. That leaves couples with queen size beds. Most such people are not youngsters. They have climbed the career ladder a rung or two but are not at the top. They have splurged to upgrade their double beds to queen beds but find that they are not as young as they used to be. They are not old but are still starting to feel their age. They wake up in the mornings with stiff necks, aching backs and sore muscles.
These (late) middle-aged couples often start out by looking for better pillows. It is a poor salesman indeed who cannot persuade them to buy memory foam pillows (or some other high-end pillow). From there, it is just one more step to adding a memory foam mattress pad to their order. This is a scene that is often seen in furniture and mattress shops everywhere, everyday.
There is also a second atypical group of people who buy queen-sized memory foam toppers. These are usually youngsters who have decided to skip the idea of sleeping in a bed and jumped straight into sleeping directly on memory foam topper. Basically, they are treating their toppers like Japanese futons: unroll it and sleep on it at night, roll it up for more space after waking up in the morning. After all, these toppers are more comfortable than mattresses, so why bother getting a bed and mattress in the first place? And since they are already saving money on the bed and mattress, why not spend the spare change on the luxury of a queen sized topper, especially since it can be used much like a futon only more comfortably?
There are good reasons why queen memory foam pads are popular. If you previously preferred a smaller sized topper, perhaps this will make you change your mind.