First, it is a tactic of a cult:
Members are expected to devote inordinate amounts of time to the group.
http://www.csj.org/infoserv_cult101/checklis.htm
Second, the WTS believes that
The DEVIL finds work for idle hands to do.
9/1/88 WT p. 14 Stand Firm Against Satan?s MachinationsThe great Adversary is not idle. (1 Peter 5:8) Perhaps that is why a proverb says, "The Devil finds work for idle hands to do." He is out to subvert all genuine Christians. (2 Timothy 3:12) And he can concentrate on Jehovah?s people for one simple reason?he already has the rest of the world in his power!
12/1/93 WT p. 27
Keeping busy in Jehovah?s work is the foremost way to oppose Satan and his world and give Jehovah exclusive devotion.
8/15/90 WT p. 15
As loyal servants of Jehovah, we must keep busy in his service and be guided at all times by his holy Word. This will help us in our firm resolve that no price Satan can ever offer will swerve us from loyalty to God.
The work of preaching is now taking on added urgency as the end of Satan?s empire draws near. Having the truth of Jehovah God?s Word, as his Witnesses, we have an obligation to spread the good news at every opportunity. The work at hand is more than enough to keep us busy, right up to the end. We must never allow ourselves to be sidetracked by hedonistic, immoral pleasure-seeking or to be weighed down by materialism. We should not get involved in speculative thinking, debating about words, for this can prove to be unprofitable and time-consuming.
Blondie