And how should they increase the difficoulty?

Monsters should deal more damage, or have more HP, or just be more of them...

All the re-tweaking and testing, and further tweaking and testing of the already existing content (also act 2-3-...) for 3, 5, 6 party members would be a lot of time.
And if they implement it, then people would start complaining about how they implemented it, and why it should be done differently...