Many Witnesses are afraid of serious Biblical questions or objections being raised by the householders. Part of it is from an obsessive fear of "apostate-like" people; but some of it is from being ill-equipped to discuss doctrinal issues using only the Bible.
How many JWs can have an in-depth discussion with a householder on the topic of being declared righteous through faith in Jesus, for example? How does a JW explain (using only the Bible) that one must declare that they are being baptized into an organization in order to qualify for Christian baptism? 1914 chronology creates migraines for most JWs. How do they explain it using the Bible? Many cannot. This is why the Reasoning Book is so crucial. But whipping out the Reasoning Book to try to explain a doctrine that is supposedly well-grounded in Scripture creates dissonance for some. And to most people at the doors who know a lot about the Bible, it looks idiotic and cultic.
The door-to-door work does require salesmanship, whether they like it or not. It absolutely requires that one be able to verbally tap-dance around troubling organizational issues. This is very tiring for many JWs.
It is not Satan and the demons making people tired and weary of the preaching work. It is the energy they must expend, and the sophistry that they must employ to sell a new gospel that is the problem.