If all schools were 2 sessions then there would be some sort of consistency within the educational system and this would allow for greater utilization of time and resources, not to mention that this 2 session program would benefit the students as well giving them a better chance to learn more during this time.
Some schools don't follow the two semester method, at least at the college level for higher education. Many public schools have summer school to get kids ahead for the next grade. Some schools implement block classes, meaning they only go for a few weeks instead of an entire semester of 17-18 weeks. The semester or two-session system doesn't always apply.