I also think it's very possible that clinical programs tend to get a higher number of completely unqualified applicants. You know the type - they get out of college and don't know what they want to do with their lives and then think, "Gee, I like talking to people. I'll be a psychologist!" And then they apply with only OK GPAs and GRE scores, no research or clinical experience, and, in some cases, no psych courses. And of course they don't have a prayer of getting in, but they still ramp up the number of applicants.
On the other hand, I think most people applying for developmental programs have done a little bit more thinking about whether they want to devote their lives to doing research and have probably had some sort of research experience. And school programs just aren't as well known, so people may need to spend more time learning about the degree and whether it's right for them. Just spending some time thinking about whether a graduate program would be a good match makes them a heck of a lot more qualified than the "I'm going to be a psychologist because I don't know what else to do and I'm applying to UCLA because I like Southern CA" types.