I have less experience with Romanian culture than he, but there might be a more universal collection of reasons why people might not trust Oliver Twist.
There's the correlation of bad luck and bad character: the bad things that happened to his parents are because of bad things they did, and the lad is going to inherit the bad luck. Keep him away from me.
You can't know an orphan as well as you can know a child when you know his parents. Knowing the parents gives a strong clue as to which unfledged traits are going to be important in his character. In addition, you can tell from them how he has been/will be trained. Without them he might have been trained by Fagin.
Without parents to tie him into the extended family and tribe, is he going to be part of the tribe? Will he be loyal? (OK, this one is a little circular, since if you are part of their tribe you could take over the office of tying them into the tribe yourself, if it occurred to you.)