Bar position has certainly been a subject of fashion over the years. In the olden days I've gone from 80's 'short' stem and riser bar which became uncool, to the completely dumb 150mm flat Syncros stem with cut down Answer Hyperlite which was good on gravel climbs but nothing else. The riser bar bike was a cheap Canadian Tire type bike (Halfords) and had relaxed geometry and long stays which was 'old fashioned'. The funny thing is, there was a steep rooty tech climb on my local hill and I could just about clean it on the cheap bike as the weight distribution was pretty good. On all my 'cool' bikes with short stays and stupid stem / bar combos I could never clean the climb as the bikes were unbalanced, and you'd just flip off the back of the bike.
And then in recent-ish times there was the '800mm flat bar on a DH bike' fashion. What on earth was that all about? Was is a desperate attempt to get as much effective reach as possible as the bikes were so short? Or did some DH racer tell everyone to do it, and like sheep they did, like having completely horizontal brake levers now? ;)
I haven't seen it mentioned (but could have missed it), but I think wheelbase lengths and chainstay lengths are pushing people into higher handlebars. Going back to my 80's bike, longer stays and wheelbase gives the bike a more planted feeling on the climbs, so one can get away with a higher bar for better positioning on the downs without wheelieing on the climbs. But also in order to get the front wheel off the ground with subtle body movement one needs a higher bar anyway, so it's a win / win. A higher bar also brings back a little of the 'playfulness' that has been lost with soccer pitch length bikes.