What bull is this? TIE fighters have been modified to carry Proton torps. TIE bombers carry them on a regular basis, TIE Advanceds also carried them, as did TIE g/t. TIE bombers are actually as fast and manuverable as Y-wings, they aren't as bad as they are made out to be. Plus, the Empire has loads of themThe thing is none of the iperial engineers recognized the exhaust ports weakness,
also they woud need to fly a heavily modified tie bomber into the trench for anything approaching proton torpedo level power,
a difficult task considering how clumsy Tie bombers re.
Proton Torps are standard tech. And if we go with the Lemelisk theory, the Empire already knew about the exhaust port and didn't think the Rebels would find it. How would they know the Rebels would manage to get ahold of the plans.