Part of being a college student is being the target of many emails about surveys, studies, and other students' psychology projects. In general, I suppose, it's easier to get college students to participate in a research study than a more representative sample of the human race.
Imagine the scene: you have access to the student mailing list and your study about how carbonated drinks affect concentration is all ready to go. Out of the hundreds or even thousands of students who will get an email, how do you get twenty or fifty to show up? If you have a lot of friends, you could ask them personally, but "n = my friends" would likely not go well when defending your thesis.
The answer, of course, is incentives, and money is the most effective (or Amazon gift cards, which are essentially money at this point). The larger the incentive, the more representative of the population your subjects will be-- if you're asking for volunteers, you will be studying people who like to volunteer for things, but if you offer $100 per hour, you will be studying a much wider variety of people.
The problem, then, is that research funding is limited. One common work-around is entering participants into a raffle (for an iPad, for example) instead of paying each participant individually. Whenever I see that the incentive for research is a raffle, I assume that it's less expensive for the researchers than paying people individually. I'm not generally optimistic about my chances of winning a raffle, so I usually just ignore any studies that leave most participants empty-handed.
I recently got an email about a psychology study with both fixed pay (five pounds for half an hour) and a raffle for prizes of up to fifty pounds. Usually, I would consider this a good deal, but the study involves completing speed and accuracy tasks while receiving "mildly painful electric shocks". I suppose you could luck out and be in the control group, but I don't need five pounds quite enough to take that chance.