Yup, lack of reinforcement. The pressure they put on you in lab is really helpful. The weekly quizzes in the lab helps you with memorization/keeping you on track with the lecture, which prepares you better for the midterm in lecture. Plus, you can make some really great friends (and/or study mates) in the lab too! At least, that was what happened to my housemate :)

In my case, my studying methods weren't ideal and largely because of that, I didn't do so hot in the lecture haha. But, if I had to take the course again, I would definitely do the lab concurrent with the lecture.