Several reasons for avoiding use of the modern media in spreading what they purport to believe is a message of the utmost life-and-death urgency: for one, the GB must sense that such methods would place them in a public forum where their core beliefs would be vulnerable to exposure as indefensible. That's explains the uniquely reclusive character of the Governing Body; in contrast with leading figures in other religions, it would be unthinkable for any of them to grant interviews, much less debate, in public media forums.
Then other reason is that for them, the door-to-door preaching serves as a "busy work" treadmill for the masses of JWs, which keeps them servile and in line; and at least temporarily assuages the guilt feelings heaped upon them relentlessly by the "do more" drumbeat at their meetings.