Why do fathers abandon their kids once the mother ends the relationship? Is it because she broke up with him? is it too hard to see her again so they don't see their kids?

You're trying to lump everyone into the same box. Everyone's situation is different.

Sometimes the mothers make it difficult for the fathers to visit, trying to cause the father to suffer because of the divorce. What that does is hurt the children and teaches them to try and get back at someone instead of working it out. Some fathers choose to stay away because of the tension his visits cause.

Perhaps there's drug / alcohol abuse involved and the mother is wanting supervised visitation to make sure the kids are kept safe.

Sometimes the fathers just don't want to bother. Don't lay all this at the fathers' feet since mothers behave badly as well, which cause a chain reaction in the splintered family.

So if your father is not wanting to see you, if you have a way to get in touch with him - ask him and be prepared for his answer.