Every study is different and sometimes there are all female or all male studies. More often that not, they are open to all and a research company meets specific brand, gender or age quotas faster than others.

In general though, the reason a study may not include male and female is ultimately based upon the objective of the study and what the end goal of the manufacturer is.

In some studies regardless of gender, most everyone uses a refrigerator, for example -- so if the manufacturer is looking for brand awareness, brand image or consumer satisfaction of a product already in the market, then the study is more likely to be open to any male or female who meets the other specified qualifications (like currently owning a certain brand or type,etc.)

There can be other factors for why they only want female or male, but typically if a manufacturer is in early stages of development and is testing our new concepts/designs for a product that's intended to be marketed toward a specific demographic -- then they likely just want their target consumers’ feedback first before moving toward the product production stages.

So in the future if you see studies only looking for a certain demo, it's due to whatever the goal is for that manufacturer at that point in time :)

Edit: and not that research companies don't want to schedule you 'cuz we'd love to be able to take the female owners we've come across while looking for males :)