There's likely multiple reasons for this. One of my gripes with Medium is it's lack of transparency for how writers earn money.

If your views are coming from mainly external sources, Medium won't pay you for that.

Also, Medium pays more for more recent stories. Probably as an incentive to keep posting more articles. 1 hour of readership for a brand new story could get you $2-5. For an older story, it'll probably only earn you less than a dollar.