I think that insofar as JWs actually believe the Watchtower teachings, it's because of the constant repetition along with the constant putdowns of alternatives as worldly and satanic.
The constant indoctrination leads them to shut their minds off when anyone tries to show them the doctrinal flip flops and false prophecies.
Many years ago, I tried to show an elder some of the ridiculous statements the WT has made. I used Randy Watters' "Thus Saith the Governing Body of Jehovah's Witnesses" booklet.
He refused to look at the evidence because of the source -- "apostate material."
I tried to get him to check the originals to see if Randy's photocopies were of actual WT publications or apostate forgeries, but he simply shut his mind down and stalked out of the room.