Budgetary constraints: a field trip may require travel from one region to another. This means the school or parents will have to take care of transportation costs and meal costs. These may all amount to too much expenses.
Safety concerns: when students are out of school, they may come into contact with harmful substances like toxic chemicals or hazardous equipment. Keeping them safe from these things may present quite a challenge.
Student behavior: on field trips, students normally get a sense of freedom which may lead them to misbehave. This can create an unpleasant experience for those in charge and those they interact with.