Alex Savage and PotD are different type of contents though:
1) Alex requires a fixed party composition - PotD does not. You'll never stand in A11S with 7 tanks and a ninja and be like "oh yeah, shit, eh, you, paladin, you can heal, right?" - one of the great and unique things about PotD is that you can go in with any party composition you like. Putting the higher floors into DF would open the doors to change that (I remember reading countless stories about the recent potd, or even 1-50 with people telling stories like "I entered 4 times today, we wiped 3 because we didnt have a healer") - also: Alex requires you to find 7 - and always 7! - more people, while PotD allows you any number between 1 and 3 (or you can even go in alone...)
2) Randomness: As every endgame content Savage is very scripted and you react only to things that you know are going to happen etc. You can learn those scripts for yourself and as long as everyone in the party knows them, there is basically no communication required - look at how easy it is the do Sophia Extreme through DF because you'll always get the same party composition (the one you need for that content) and it always repeats itself. PotD isnt like that - you can have pats, traps that lure more enemies, you dont know what your map will look like etc.
3) Consequences for a wipe: There are basically non for a raid, you just start over, lost a few minutes of your life and can go again - you might even have improved! PotD past floor 100: your progress is lost, potentially several hours and, if you didnt wipe to a boss that you havent seen yet, you didnt even learn something new or made any progress (like how you do when you do savage and you get the boss to 50%, 30%, 10% for the first time and can at least say "yep, we made progress"), nope your progress is just LOST
TL; DR:
Savage is about learning a challenging fight; PotD requires you to react and adapt to "unexpected" events happening