Because divisional playoffs and conference playoffs both follow some sort of logic - the top seed plays the lowest seed. Teams are being paired off willy-nilly based on what division they're in, standings be damned, with any mismatches getting paired off because they have nobody else to play.
Such a system of determining playoff pairings would have the same logic as determining them based on the skate size of each team's starting goaltender.
...and your scenario above implies that your 3rd-ranked team, for example) would still be ranked third if they were in the opposite division, despite having an entirely different schedule.
I've been reading that a lot recently? And again I ask, How so? In a more balanced 6-Division structure, every team would play every other team at least 2 games, and in a 4-Division schedule, after the 1st Round, won't half the teams have played a "different schedule"? The only important factor is that all teams facing each other in the Conference play each other an even number of games. But even at that, at least with the current system, you play against Conference opponents that you faced a minimum of 4 times in a Season. In a more balanced schedule, with 4-Divisions, the 2nd and 3rd Rounds will be with several matchups of teams that only played each other 2 times in the Season.
You're sacrificing the 2nd and 3rd Rounds in order to get teams that played each other the same number of times in the 1st Round only. How is that an improvement?