While I'd never seen any statistics, it didn't take me long to notice that everybody around me at the KH was an ignoramus, an idiot, and/or a broke-ass loser - with exception of a precious few who had achieved some kind of life before their indoctrination. You can bet someone raised a JW is a complete waste of oxygen.
From what I've seen, there are just not many thinking, religious people among JWs. One of the many things college presses a student to do is think; not just blindly obey. They're taught how to research, how to (and not to - e.g. logical fallacies) argue for and against a subject, to think outside the confines of and expand on previous information, and to question and require proof before adopting a position. For a centralized, power-hungry cult, this is a path that can only lead away.
Their education bashing is actually an intelligent move. While, granted, the congregations may be filled with mouth-breathing dropouts, at least there are butts in the seats. While their scant wallets may only donate a pittance, it's better than getting nothing from those who leave because their brains function properly. They're also "useful idiots" in increasing the membership.
Born-in JWs are forbidden to think or read anything the WBTS didn't print, from infancy. Converts are generally idiots from similarly-stupid religions (evangelical "God hates fags" types), other apathetic born-ins (Catholics or geographical default religions), or those who had never given religion a second thought.
This last group may have been put off by the pageantry of Catholicism, the hocus-pocus of the holy-roller tongue-speaking snake-handling "charismatic" religions, the mystical beliefs of immortal soul, trinity, transubstantiation, Hellfire, etc. The JWs present these folks with a pretty straight-forward religion by people in business attire rather than brocade robes and enormous hats.
While this last group may be thinkers, they're rarely religious or spiritual. They really don't doubt what they're being told because they don't much care and never did. They're not Bereans and they don't fact-check what they're being told. They never knew some of what churches taught wasn't in the Bible until JWs came and showed them. Likewise, they'll never know some things JWs teach isn't in the Bible until someone shows them. But who's going to? One of their "friends" in the JWs? Hell no. Some other religious knock on the door?
These are the ones the Bible parlor tricks JWs like to pull at the door work on:
Q: Do you know God's name?
A: No, I never thought about it.
Q: Do you know what the "kingdom" we pray for is?
A: No, I never thought about it.
Q: Do you know what people in Heaven will do?
A: No, I never thought about it.
etc, ad nauseam...