It must feel awful when others see you as a bad parent. And yes, people are especially quick to judge parents in particular.

However I would like to do some sober reasoning while keeping in mind that this is a sensitive question.

To understand why people are judging you must look at evolution. It has been concluded that judging behavior most likely has evolved within societies in order to keep them together. By punishing people who causes damage to the group, it would be better off.

You say that it is nobody’s business if you have a child when you’re poor. But it is. If society lets anybody, regardless of their means, who wanted a child to have it, then there would be a lot of children that society would have to take care of. Especially with the welfare systems in rich countries.

It hurts a great deal to be judged by the rest of the society. It is possibly the worst feeling one can experience, worse than physical pain.

But this might answer your question as to why people have to judge. Stop seeing yourself as a victim, and focus on earning the respect from others, starting by admitting that you had poor judgement (nothing bad with that, everybody makes mistakes!), is the first step towards avoiding getting bitter.