People look at older models as older models, even though they may be good enough...

People look at older models as older models, even though they may be good enough and may provide software updates for longer compared to many Android phones in that price range. It also looks odd (to the purchaser) when one is buying something that's a four year old model. Android phones, with the range of hardware differences, don't yield to such simple comparisons, and many of them offer slightly better hardware Year on year at the same price points.

Apple probably needs to come up with newer, refreshed (hardware), low priced models to beat this. But I wouldn't hold my breath on such a move happening.