Because the NFL is run for the most part by idiots. Even Jon fecking Gruden could see Mahomes talent, but apparently NFL scouts and GMs who picked before KC are stupider than him. The bears traded up to #2 for Trubisky.
Which has paid off nicely by winning the division. Ultimately, no one could've guessed Mahomes would be as good as he has been since most of his predecessors from the air raid/spread/run&shoot etc struggled in the pros.