Don Durito
Hero
I think there's a coupe of things behind it. One they've doubled the hitpoints effectively from 3rd edition by introducing hit dice recovery, so that effectively doubles the number of encounters to really put pressure on the hit points (and healing magic). If you remove both cantrips and hit dice recovery you have something that looks a lot closer to the 3 to 4 encounters assumed by 3rd edition (although I don't think I'd recommend it necessarily, the game was finished off with the assumption cantrips and hit dice so you'd probably get a few weird things.)So one of the first things you learn in playtesting is how the user actually uses the tool you design. Not how you envisioned, the actual.
It has always seemed to me that 6-8 encounters as the normal in a given day is just incredibly high, and its not a model I have ever seen used in any game I have played in or run. I have asked my friends if they have ever played in a game where that was normal...and people in general have said that is very very high.
Now if we really are the minority, so be it, it is what it is. But if its true that most players simply don't play the way that the designers intended....the fault is with the design. You don't get mad at the user for using your product in a way you didn't intend....you change the design to better guide them....or accept that the way they use it is fine and design around that.
So if the average group runs less than 6-8 encounters as standard....than balancing the game around such numbers is a design mistake.
The other thing is that 6-8 encounters is not really all that exceptional in older editions (pre 3rd edition) with dungeon crawling. You could easily get through that number of encouters in a single session of a B/X game. But there's some key differences which cause problems when translating that to 5th edition. The first is just the amount of time that combat takes (A random encounter with 3 skeletons is over to quickly to really be annoying). The other is that 6-8 encounters is with everything going well. In B/X a few bad die rolls in any of those encounters may mean a dead PC or at least the need to retreat for the day (Meaning it's really more like 1-8+ encounters). Easy encounters don't feel worthwhile in 5e because it doesn't feel like anything's immediately at stake. The critical hit from the skeleton in the first fight of the day can in practice make the difference between success and failure in the final combat of the day when all resources are drained, but that doesn't work psychologically. (And it especially doesn't work if it takes you three sessions to get through all those 6-8 encounters - not just through length of combat but all the additonal things like talking in character and interacting with NPCs which mean that modern games move more slowly then old school ones).