A better question is, why do JWs only stress the commandment to preach when Jesus also commanded his disciples to do charitable work? I guess ACTUALLY helping people is a waste of time. It's better to knock on doors at 10 AM on a weekday when no one's home...
On topic, I've read Acts twice in the past few months. It seemed pretty obvious to me that they were mostly preaching in public and only preaching in homes when they were the homes of other Christians or people interested in Christianity. Jesus certainly wasn't preaching door to door. He was always preaching in public, mostly in synagogues. I was telling my JW boyfriend that if he really wanted to emulate Christ, he'd go into a Mormon or Catholic service and start debating with deacons and priests.