hmm i think this might be due to the fact that nowadays, most of us, in general, are used to these super hi fi recordings where you'd get this full heavy punchy sound once you glue a bass sound together with multiple guitar tracks and lots of compression and heavy bottom .. the choice is yours..
with downtuning you get that aspect of substituting a bass guitar player (or at least partially by moving closer to the lower fq spectrum as well as achieving some new aliquotes to pop up).. in an open space like a shop, or a bedroom or in an empty practice room you'll notice that you somehow tend to run the presence and treble down cause you do not need to cut through..you're aiming for a fuller sound...even though the amp does not play differently, it is ones perception of sound moving through space that makes us feel these differences..
Over the years I've come to the conclusion that usually sounds that tend to appear at their best in a band mix are somehow not so appealing as those that one uses playing on his own (i did not use the word 'her' cause you know, girls, there's the kitchen) . at least with me.. I've seen so many guys who just prefer to play with scooped mids on their own cause they spend most of the time playing in bedrooms, empty practice rooms and prefer it that way.. once they hit a full band, there are these topics appearing that start as 'i cant cut through.. is my amp broken?'
sometimes I feel like most of the people around me downtune their 6's cause.. if you can't play one normal thing that would make sense, you may still hit just a simple open chord it will shake the walls at least and sound evil.. imagine doing it on a standard E tuning.. NOONE CAAARES BROO!

just kddin'
I once had the chance to listen to a multitrack by Lenny Kravitz where in the original song there was this superb guitar sound that just sat there like a mofo.. when I listened to the track itself I could not believe the difference it just.. sucked..
but that's just my theory

at the end of the day, it's about fun and doing things that make us happy, whatever suits us all best