They needed to keep preaching the end or else people would start to lose interest. It's a shitty religion to be in, so unless you see yourself as a savior of mankind it's tough to keep up the effort.
Being a JW kid is the worst. Little Johnny has a birthday today? Go stand in the hall while everyone eats cupcakes. Oh, it's Valentine's Day? Sit at your desk quietly while everyone exchanges candy. Knock on a door to hand out Watchtowers and your schoolmate opens the door? You still have to give your sales pitch or else the elders will counsel you.
And then there are the sermons on Sunday morning about the sin of masturbation. Turn beet red for one entire hour thinking that they are talking about you specifically.
Horrible.