OP, to actually give an answer to the original question you asked (why users keep posting the same 5 anime even though they're "all supposed to be into old shit" and "there was other anime made during that time" (your words)), one I can think of is that those are the ones most users here like the most. There's probably a 100 other anime, but since most users dont like them as much as those main 5, they don't talk a lot about them unprompted. Imagine asking what sodas people like drinking the most. Most would probably say Coca-Cola or Pepsi! You probably won't get an answer like luzocade or jarritos that often.

If you want to get more people talking about other old anime, make a thread and anyone interested will reply! But you can't force users to talk about something they don't like or enjoy watching.