Preaching isn't meant the way the JWs would mean it. How would that occur, anyway? We can't actively seek out JWs and knock on their doors. There are Reddit and quora threads, but there are veterans on both sides who aren't going to be easily swayed by the other side's arguments.
Occasionally someone with doubts will find this place and ask advice, and the exJWs who've been through something similar can provide that advice and some encouragement. The "preaching" that happens here is therefore more reactive than dynamic (hat tip to Vidiot).
Also, it is quite possible to be entirely passive and the anti Watchtower message will get through. This discussion board has been around for decades and has a vast number of threads on pretty well every topic that a questioning JW could possibly have thoughts about. The search function is pretty decent as well.