It's definitely not "impossible" to create Fighter under 3E rules that is good at Diplomacy. You just have to value CHA and Diplomacy as much as you do STR, CON, and DEX. You put your highest roll into STR. You put your second highest roll in to CHA. Third highest goes into CON. And fourth highest into DEX, etc.
Um, note that Diplomacy is not a class skill for fighters, and fighters don't get many skill points to begin with. And with that arrangement your Int score is your 5th priority, so not many extra points from that. I don't think you'll find the character actually all that good at Diplomacy in the long run.
I think the better answer is: guess what, D&D is a class-based game, and yes, staying pure in a class limits your options! Go figure!
There's a fault in creating the concept of , "a Fighter that is good at diplomacy". Fighters are, by definition, bad at diplomacy, but good at fighting. If you tie the concept to the class, of course you are going to be limited. If you decouple the concept from the class, and say instead, "A character who is better than average at fighting, and has some diplomatic ability", that is well within the ability of 3e or 4e to build.