In many Kingdoms the King and Dukes are Mates, they then recruit as many players as possible, then when it suit them they kick the players from the Kingdom and start farming them. I guarantee you that this selfish and Mafia type of Play result in 3 out of 5 new Players not playing the game again..
Personally I have seen this happening and I was even a victim of this type of Play by Idiot Self Centered Players.
While I agree that this is what turns away new players from wanting to stay, I disagree that it is entirely the fault of existing (experienced) players. There is little to no incentives for having a new player in your kingdom, who will provide little to no contribution overall to your teamplay. Teaching them takes a huge amount of time and effort, and even then it does not guarantee any pay-off in the end.
The game is simply designed in such a way that it is easy to take up and understand the basic concepts (which will at most teach you how to be a good farm for others), but to understand and apply the advanced concepts take a lot of time and perseverance from two parties - the experienced players who will have to explain to them, and the new players themselves who have to be willing to commit and learn.
Honestly from my own experience most of the time the problem comes from the new players, who simply do not wish to try to gain a deeper understanding of the game. 90% of experienced players actually do not mind explaining how the game works (I would say they actually enjoy sharing their knowledge).