Delta doesn't have any MAX aircraft - and I'd think most people who care would probably know that, so I doubt that was a major part. Seat wise, Delta configures for 132 on an A319 and 180 on a 737-900 - I doubt there were more than 90 people on the return flight so there's more to it than that.
What WHBM wrote is interesting and plausible - it's just not the way we did things when I was working. We avoided flying on Sunday - messes up the weekend and we didn't get paid to fly on weekends (engineering - the machinists did get paid to fly weekends so they didn't mind as much). With laptops, we could still do most of the stuff we could do in the office while on the road, so not much rush to get back to spend Friday in the office (I used to like to stay at places like Embassy Suites that had evening receptions - I'd go down and drink beer while I worked through the day's email - made the email go much easier
). So we'd typically leave to head home on Friday - with the 3 hour time difference between the east and west coasts, we could fit in nearly a full day on Friday, take an evening flight, and still get back to Seattle at a reasonable hour and have a full weekend.