One reason why it's said so often among blacks is the perception that low-class whites are still white and therefore have a chance to make it for themselves. Rightly or wrongly, many blacks do not believe the same is possible for blacks and so low-class blacks are more in need of "help."
At the same time, many, many blacks have washed their hands of lower-class blacks so the above isn't as widespreads as it once perhaps was.
Maybe the help is actually doing them more harm than good. At least for the whites in the same position, they pretty much know no one is going to come along and help them -- if they want to be helped they're going to have to do themselves. No one feels sorry for poor low class whites, just like the blacks, whites also figure they have a chance to do it on their own. Once you become one of those "my people" who someone wants to help, you're being turned into a helpless, dependent type.