Generally because you do not have the time to waste and you do not want your mind to be bogged down with too much stuff which it might be tempted to use and doing so gain bad habits which might cost the students his life in a confrontation. In 3 weeks of IDF Intensive course you get the core curriculum of 21 hours regurgitated through lesson plan evaluation about 3 to 4 times. So you would get about 144 hours of Pure Krav Maga and the rest is warm up and fitness. A soldier Trainee day at the Fighting Fitness Academy is longer than the classic 8 hours work day, however still the learning factor is taken into account and sufficient rest is required. Warm up and fitness can be done in early morning and late night hours giving at least 8 hours a day for Pure Krav Maga and plenty of time to afternoon rest after Lunch!
In civilian Krav Maga of about 480 hours in biweekly classes in five years, assuming students take off about one month vacations. You need to divide the 480 by 4 because for each hour taught you need to do warm up and fitness which takes 20 minutes each and for the rest of the hour you need to consider that some people do skip classes and there are many levels of people in the class. So you really get about 120 hours of Pure Krav Maga which is about what you get in the Israel Defense Force Krav Maga.
However, you do not get the lethal elements and since you spread it over 5 years the retention of the breakdown of the training system is generally approaching zero! Therefore, the answer has to something to do with prioritization that students will learn how to handle themselves in reality based training than be familiar with more and more tricks. So the limited Pure Krav Maga Curriculum was carefully selected and had a more profound reason behind it.