To those in the know:
Why do many of the DC-9 flight numbers through DTW have an "equipment change"?

As an example today's flight 1402 from BNA-DTW-IAD has an equipment change along with the nasty A9 to A64 schlep. It seems that most of the DC-9 flights to/from the DC area do this ... and perhaps other places as well.

Why not just keep the same DC-9 for the flight numer? Or why not just use different flight numbers ... one for each aircraft?

TIA