You are not allowed to bring on large quantities of liquids or gels because they can easily be flammable or explosive substances, disguised to look like juice or toothpaste.
And that, of course, is the dumbest reason ever because if it takes 9 ounces of a certain fluid to be explosive, then the terrorists will put it in 3 3oz bottles and combine them on the plane. Terrorists know how to do basic math. The liquid thing is security theater.