I started a new project around the PHL area and am trying to continue to fly Delta as I prefer them over all other domestic airlines. One issue I've run into is the Thursday evening return flights to DTW (where I'm based out of) are priced incredibly high on my travel site (I'm looking 3-4 weeks out from today as I did expect immediate flights to be expensive).

I went onto Delta's site and found out why - they're only selling M fares on the route. At first I figured that this was just cause the flight was close to sold out but after looking at the seat map only 9-10 seats, at most, were marked as occupied...

Is this normal? Should I wait until a different date when more fare classes will open up? Just seems like I am missing something and apologies in advance if it is something simple.

If anyone wants to look at specific route for reference, I'm looking at the flight that leaves at 5:25 pm Thursday March 12th from PHL to DTW.