Why does DL use older equipment on longer flights?

I am MCO-based, so I go thru ATL alot. Usually on newer 757s with updated media systems. Lately I have taken the opportunity to avoid ATL by connecting in DTW and MSP. Surprised that these, much longer flights have older equipment. A 2+ hour flight that actually has the time to watch a movie does not have the system while the 1 hour (+/-), ATL-MCO flight has the system but not the time.

What is the rationale?