I think that had as much to do with combat duration then anything. Depending on how good the DM was at building encounters, combat could drag on and exhaust the players as much as the characters. So the players are motivated to stack as much as possible so they are not personally tiered when combat finishes.
Well, its complex. IMHO what happened is people didn't understand (and by people lets be clear I mean people with the initials M. M. basically) that 4e combat WILL NOT WORK FOR A SLUGFEST (I mean it CAN, but you have to be VERY specific and design the encounter that way to make it interesting, and then its really only good as a change of pace, maybe once per level at most). So, you got these combats that almost entirely and exclusively are straight up "steel cage death match" encounters. Both sides have no other motive except to fight to the bloody end, and no other way to end the fight except perhaps to retreat at some point (which rarely happens, and 4e doesn't have explicit morale rules, aside from Intimidate use to force surrender).
The upshot being you had to chew through the full hit points of EVERY opponent, all the time. It just isn't the right way. The PC's objective should be getting past the bad guys, or whatever the narrative demands. Maybe killing them all is decent option, but it doesn't have to be. Likewise if the bad guys are trying to SURVIVE, or achieve some purpose of their own, then the encounter will rarely turn into the slogging affair that you note. It also means that things other than mass DPR optimization start to matter. If you can really move around the map then you can get by bad guys, or snatch something, or whatever and not fight it out with each dude.
So, what happened is, the PCs were offered this slogging KotS affair, which probably 50% of all groups got inflicted on them on day one of their 4e experience. They learned REAL FAST to do nothing but jack their DPR and that every encounter would be a fight to the death for team monster. Soon the PCs are limited mobility damage output machines without much provision for stealth or anything else unless it is directly in service of doing more damage. At that point ALL they can do is brute force every situation. That's fine, but most groups don't WANT that all the time.
This is also why soldiers don't work well, because they were supposed to be impeding you from doing other stuff, not nailing you down to the floor in a slugfest all encounter. You were supposed to break past them or whatever so that they weren't an obstacle anymore. Likewise elites in general don't work well in the slug-it-out paradigm.