Probably didn't sell well. They released a newer Blu-Ray version, in which each set has more episodes at a similar price. They've released the whole series that way.
I watch multiple series. However, if I want to see series that are long running or have ended, I usually keep it one at a time. I still have series that I'm caught up with, and whatever interests me when a new season starts. It'll be too much if I add more than one series to finish or catch up with.