in no prticular order: weight; cost; testing required; certification; structural issues; size of the parachutes required; fact, that most commercial aviation disasters happen during takeoff and landing; strong possibility of it catching on fire upon touching down; risk of it actually hindering the safety of the aircraft during normal operation; engineering challenges connected to, among other, wiring, ventilaton, pressurization, hydraulics, fuel lines; the fact, that we don't live in a dahir inshaad world
dunno