Field pants are specialized to protect against the elements, often focusing on a specific problem, without lowering your mobility, agility, or combat ability. They are made to be light but durable.
Tactical pants are used as to allow maximum combat and operational ability to the wearer while still protecting them from the elements and allowing them to carry gear and other utility items.
Unlike casual clothes, tactical pants are not made for appearance but for performance. They should be comfortable, but allow for unrestricted movement with their cut.