Because in time you are no longer able to think. They tell you how to handle every situation without using your brain to come up with the right way to handle things.
You get casted away and loose your feeling of self worth (assuming you end up actually believing it all) if you make a simple mistake. No one ever helps you with your problems. They just disown you if you have them.
They discourage so many things that don't make any sense --- like using the internet, having wind chimes or dream catchers, saying "bless you" or "good luck", joining a civic club, doing community service, giving blood, taking blood transfusions, going to college, joining a sports team, eating lunch with a coworker, or even having a friend or spending time with a friend who's not a JW.
You loose having a relationship with God even though you are a religious person because you view God as a big mean angry guy. They take out Jesus from worship, and don't really encourage or pray for others during hard times. Their love is conditional and will end the minute you disagree with one doctrine. You must believe every single thing they teach without thought or question. -- Leave your brain at the door.
Their doctrine is not what the Bible teaches and is pure heracy. Just the point that you would be believing a lie is enough.
They are false prophets. They have predicted the end of the world and been wrong several times.
Their practices have even resulted in death. Their erronous teaching of no blood transfusions has had thousands die each year. They have let people be murdered for their lie in other countries (Malawi / WWII)
If you were to be raped, you could not report it to the police without threat of being disfellowshipped unless there was a witness to the crime. They allow pedofiles to be loose, unconvicted, and knocking on doors where children live.