There are two main reasons why it takes so long.
Reason number one is as always money. The primary objective of any and all commercial enterprise, as dictated by law being the making of a financial profit, supersedes any other reason for pretty much anything happening in this world anywhere other, than in the minds of the ignorant. Any development of flying stuff in a world not subjected to this rule inadvertently reveals established aviation as a slow moving dinosaur of protective rules and technical conventions. You will have a hard time finding any established operator in aviation willing to question the holy 'Laws of Physics'. Even if it was out of curiosity or just for the hell of it. They do however all have an excuse for the existence of tail rudders, besides it being a nice place to put a tag.
The second reason is aviation's relation to safety. Flying being inherently unsafe due to its natural tendency to turn into falling, which mind you is not the same thing as flying down, in combination with the industries need to share this flying experience with others, has turned aviation into a true safety bastion, exceeding in this respect any other industry in which mankind is active.
Thus aviation has managed to create an entirely unique human subspecies called 'airline pilots'. These originally enthusiastic healthy human beings, employed as underpaid volunteers, are conditioned through extensive and never ending training and mental derangement, to be able to perform at absolutely superhuman levels under the most horrifying of circumstances, while being equally capable of staying in a continuous state of high alert for many hours at a time of entirely uneventful flying. The progress of technology in aviation is pressed to match their effort in being scrutinized to the very molecule in respect to safety.
Strangely the absence of working FBW systems is not and has never been one of the obstructing reasons. A current example of an available, but entirely non implemented technique, is the use of FBW systems that can adapt an aircraft's controls after damage.
Demonstrations have been made with quadcopter autonomous unmanned aircraft, that can adapt to the loss of one rotor in order to maintain some sort of controlled flight. As such aircraft are being planned as future airtaxis, these systems might result in groggy but living passengers, or even pilots, where otherwise the thing would have just crashed.
Even common helicopters could benefit after loss of tail rotor control. It is almost impossible for a pilot to successfully maneuver a spinning helicopter, but for the appropriate FBW it's just another day at the farm.