There are a lot of factors in play here. As noted above, the effects of the Hour of Code and other pushes to get more into programming won't be felt in the first year. My anecdotal situation - AP CS classes at my school increased from about 40 students to over 60 this year. I don't know if this trend is seen throughout the US or not.
There is also the issue of preparation. Not every school has a teacher who is "qualified" to teach the rigorous material in AP CS. My guess is the vast majority of AP Calc teachers have successfully finished Calc 3 and beyond, but that's not true for many AP CS teachers who might be a Business teacher or a Math teacher with a couple of CS courses. There are some schools that require students to take the AP test if they are enrolled in the course (and I believe the schools pay for them), which may account for many 1's.
Additionally, not all schools have a program that leads into AP CS. Unlike subjects like AP Calc where there is past curriculum that builds from past years, AP CS might be the first experience to coding that many students have ever had. I think efforts that are getting younger kids into coding will eventually lead to a very nice bump in AP scores in the future.
When the AP CS Principles curriculum starts being implemented more broadly, there should also be a positive effect on scores. Students who might have struggled in a more theoretical class like AP CS will gain a great deal of foundational knowledge in the Principles course. It will be a great way for kids to find out what CS is all about before deciding whether to choose the AP CS course.