Because on the line you have to deal with flight crews, flight attendants, passengers, ramp and CSM antics for one, while attempting to get a flight out while being second guessed by knuckle head supervisors and managers. You have variable lunch times and breaks. May not seem like much to you, I'm sure.
In AO you pretty much have a set schedule or pace unless you're on a drop-in line. A lot like a factory work environment. I worked in T town for 13 years in the hangars before coming to the line. Night and day difference. At the end of the day, maintenance is maintenance, but there is a difference between AO and Line, and the unions have recognized it over the years.
La Lo I'm sure will have a rebuttal.