Pricipally, because at that time, he was made the resident playwright of the Lord Chamberlain's Men, which gave him the freedom, and security, to write the magnificent works that are attributed to him. (Others had to bargain with the theatre owners to buy their plays).
By being resident in the company, he would have been there all the time, noting which actor, in rehearsal, had good diction or good deportment, (John Hemmings was a stutterer!), and he wrote accordingly, fitting the character in his mind to the actual actor rehearsing the scene. This comes out in the reading of the "script", where, Shakespeare actually records the actor's real name other than the character he is portraying!
This familiararity, made him all the more in demand, not only from the actor corps, but from the theatregoers themselves.
It was a special occasion, to witness a NEW Shakespeare play, and theatregoers flocked there to see it!