Let me preface this by saying that I wasn't in the scene yet when the changeover began, but from talking to people who were, I think the idea that people had was that lines would just naturally all switch to playing matched since they would be ABLE to with harnesses. As someone pointed out earlier on this thread, playing matched before harnesses just wasn't really an option.
So the real question is why do SO many lines still used traditional grip on flat drums when it makes no d*mn sense at all (and I make that statement as a snare drummer who plays traditional on a flat drum!) I have my opinion...I think that traditional grip is what set snare drummers apart, and without it they felt like tenor drummers whose drum made a "Rat-Tat" sound! But that is simply speculation on my part! Plus, ya gotta admit, traditional grip LOOKS way COOL!
As for being easier to play on a tilted drum, logic dictates that is is, in fact, easier to do so with traditional grip. With the drum tilted, the left hand can make a much larger arc within the natural turning radius of the wrist...on a flat drum, you have to really work on the left hand turn to get more than about 30 degrees rotation. While you do need to adjust the angle of your right stick's travel, for most players this can be accomplished very easily since the vast majority of human beings are right-hand dominant.
I've spent a little time thinking about this...can you tell? :P
And what's the result of all of my pontification? My lines still play trad. grip on a flat drum. Why? I like it.