Great question! The CompTIA A+ is split into two exams because there is too much material for a single exam. The first Exam 220-801 covers the hardware focus and the 220-801 is focused on the Operating System. I would love to tell you there is no reason for splitting this up; I would be wrong. The A+ exam might truly end up being 500 question exam if it were one exam. Since this is normally an entry level professional certification, a single exam might discourage someone new in the IT field from wanting to try it. Two exams provides for someone to study and focus on part and acheive success; then to focus on the other part and achieve success as well.
You'll see this in vendor specific exams like Micrsoft and Cisco as well. As you head toward the more advanced level certifications, you'll see the exams focus more on the mastery of the technology which means more exams(e.g. MSCE, Cisco CCNP Routing and Switching).
So does it mean more money for the organization or vendor? Well....yes and no. Let me give you an example of what I mean. A single exam can be as much as $500.00 for the each advanced exams. Where as the beginning exams may be more but be combined cost might only be around $350.00. No in the sense that your 2 exams are not given them more. What will give them more is when you seek your more advanced certifications.