The reason why Clark has to come from a poor background is so he can rise above it and get out of poverty. This is an integral part of the classic American Dream that Superman represents.
Clark's a self-made man who came up from humble, poverty stricken roots to build a solid, comfortable middle-class life for himself and (depending on the version) achieved success and (award winning) renown in his chosen field.
If he comes from a wealthy or middle-class family, there's no mountain for him to climb. If he comes from poverty and remains there, he hasn't achieved this particular aspect of the Dream.
As for the bolded part, I dont agree with that. I dont need my heroes to look or act like me. I adore Superman and I dont agree with several of his ethics (including the no-kill code). I like Ollie Queen and dont agree with all of his politics (he's far too left for this left-leaning moderate). I adore Icon and I dont agree with his Republican politics and Im not black. I love Wonder Woman and Aquaman, and I dont share their monarchy-based politics (and Im not a woman or a blonde either). I like Batman (most of the time) and Im not rich. I love Ms. Marvel and I am neither a teen, a girl, or a Muslim. I like variety in my life, and having my own views thrown back at me is boring as hell.
Just my opinion, and also no offense meant (just as no offense was taken)