I found born-in jw's are an exclusive club. They don't trust those 'from the world' and definately do keep them at arms length. I was a convert from early adulthood. (slapping forehead- what was I thinking!)
I remember repeatedly hearing comments refering to converts such as "the quicker they come in the quicker they leave", or "they'll be gone in 18 months". Converts were judged harder and scrutinised far more closely for years after converting. After you have given up family, friends, work and education opportunities and to a large extent, your identity, it's very discouraging to be treated like that. So many converts do leave and then the born-in jw's say "they didn't have the truth in their hearts. We knew it."
Also, as a convert, you have no family in the congo, no one to turn to if things go pear shaped in the congo, no uncles/fathers who are elders. Jw's know that you are on your pat malone (alone) and you have no-one to intervene on your behalf if they treat you badly. Being a convert is very lonely.
Maybe it's a result of the brainwashing that the world is an evil place? So anyone from the world may also be evil? It's only after you have been around for 20 plus years that they figure maybe your ok?
Maybe it's a result of jw's being a small religion? You find the same thing happening in country towns all over the world. For example, I understand that some villages in Britain will not accept you until your family has been in the village for at least two generations.
Similar thing in Australia (although most white Australians have only been here a total of two generations LOL), in country towns you are not accepted unless you grew up in the area. They just don't trust you and what may be lurking in your genes.