Because combat ships need to be lighter and specifically built for combat, more manoeuvrable etc. Were as explorer ships are heavier and more sturdy to with stand the riggers of long trips for longer as that is what they would be designed to do as they would be out deeper and longer on their own.
Let's think about this in terms we know: You wouldn't want to drive a Lamborghini $200,000.00 across the United States in a 4,000 mile trip. But you would be alright taking a $20,000.00 Toyota Camry. Why is that? Because exotic sports cars are tuned for extreme performance at little at a time.
So too could EDs combat ships be tuned. Simply put: Combat-grade engines could and probably should require fairly frequent basic maintenance to keep them running. Go too long or make too many jump without putting in and you put your ship at risk of being disabled, just like you'd expect a Lambo to end up being towed somewhere along the way in the trip from Miami to Seattle. Perhaps elements (wind/rain/sand) damaging sensitive engine parts and air intakes that don't exist on our lowly Camry. Perhaps the endurance run is just too long for the poor Lambo. Perhaps it simply ran at too high RPM for thousands of miles to be put on it at once.
As another analogy: Olympic track & field. There is a physical difference between the 100M dash sprinters and the 1600M runners. They're bigger, stronger, faster. But they'd also embarrass themselves if they entered the 1600M run. The distinction is in the intent of the years of training and conditioning. They are different in purpose, shape, strengths and weaknesses. Those strengths and weaknesses allow one to excel in one thing but not others.
But this is the weakness of the Elite Dangerous ship design. There are no real roles because all ships draw from the same pool of weapons and modules to fill out their hard points and internal vacancies. There is no way currently implemented to stop a FdL (the equivalent of a Lambo) from flying all the way to The Great Annihilator. And in that, there is another chance to really differentiate ships and purpose lost.
Sure I'd be happy if they went in and retooled every ship to have a limited number of loadout options. Maybe the Anaconda/Cutter/Corvette have to choose from torpedoes and turreted weapons. Perhaps certain small ships are fixed weapons only but more powerful than their contemporaries and can get far faster. Maybe some ship has access to an Ion cannon that can disable engines. Maybe a mining ship is the only ship capable of landing on asteroids and deploying what looks like a giant drill and extracting minerals.
I could go on and on and at this point it's just spinning wheels, right? Because it's too late to change the core design of the ships. Or, too late for FDev to probably consider it. But yes, people like classes in games becuase it gives purpose. Clear and distinct purpose to characters or in this case, ships. Commanders would have the ability to judge whether they may be able to win a fight purely based on the ships that have interdicted it, rather than "oh suprise!" it's one of any of 50 weapon and mod variants.
Sure, many would appreciate class definition and clarity. But we won't get that with ED unless there are some significant changes among the designer team. So why keep spinning our wheels hoping for something other than what we have? There is nothing that will be added that won't be applicable to every ship because at the end of the day, every ship has modules and hard points...and everything you can add to a ship runs through those. That's how FDev likes it. Maybe Star Citizen will give people the class differentiation they seek in 2018 or whenever it comes out. I don't know. Until then, I'll just live with ED and hope for the best.