Why do iPhones go down with a new entry?

I see different smartphones coming up day by day in the market. We now have a long list of smartphones to consider when we think to buy a new smartphone. One surprising thing for me is that why the Apple iPhones go very down after a new release in the iPhone series? I am observing this from last 2-3 years that whenever there is a new release in the iPhone series the previous entry goes very down, the price goes very down. This has been specially noticed in the Apple iPhones. I consider that the previews entry goes somewhat down with a new release in the same series in every field either in smartphones or in cars/motorbike etc. But Apple iPhones goes very down as compared to other smartphones. Why is this so? Why this is not observed in other smartphones?