12/8/2020 11:45 AM
Edited Date/Time: 12/8/2020 12:01 PM
Some additional thoughts that I haven't heard yet:
1. 29ers have taller front ends. Someone mentioned that as reach lengths got longer, WC pro's started lowering their handlebars. That's actually not true. It might look like pro's front ends are coming down because we see lower rise bars and fewer stem spacers, but that's because with 27.5 & 29" wheels, frame stack heights have gotten higher. A 29" front wheel and fork is 2" higher than a 26" frame and fork. That's part of the reason why a lot of companies are speccing stupid short head tubes on their 29er frames. There was a trend in the mid-2000's for WC pro's to run low bars (Sam Hill famously took the top cap off his headset to minimize cockpit height), but bars went up in the pro ranks around 2010 and they haven't gone down since. Going downhill, taller really is better. Modern 29er downhill bikes might look like their bars are lower, but they're actually as high or higher.
2. Good descending position depends on where you ride. As discussed above, the steeper your trails are, the taller your bars need to be for descending and you'll just need to put up with steep bars on your way back up the hill. But the inverse is that, the flatter your trails are, the lower you need your bars to be to weight the front wheel and give you a reasonable range of motion over the front of the bike. To answer the OP, if trail/enduro/all mountain bikes were only made for gnarly riders in Squamish, BC, the stack heights would be higher. But bike companies also sell aggressive full-suspension bikes to Jerry's in Indiana and to old/slow/ignorant riders in my neck of the woods who choose to compromise their descending position to get better pedaling characteristics. For a local example, if you're selling a 160mm travel enduro bike to someone who only rides at Duthie, they're going to be pissed about a high stack height because... Duthie is flat.
3. Inseam height affects bar height. Riders with tall legs and short torsos will need higher bars. For instance, Jack Moir runs short frames with really tall bars, and so do I. I'm 6'3" with long legs and I chose the 490mm reach XL Megatower over the 515mm reach XXL frame. I'm happy as a clam with that decision. Riders hinge at the hips to reach the bars, and the higher your hips, the higher your bars need to be. The opposite is true for riders with short legs and long torsos- they will need lower bar heights. Our recent senior men's world champion is a great example of this body type, and as a result Reece Wilson runs low rise bars and no stem spacers on his Trek. If you're a frame manufacturer, is your size large frame designed for 6'2" riders with long legs and short torsos who want a long head tube, or is it designed for 5'10" riders with long torsos and short legs who need a short head tube? It's made for both, so frame manufacturers play it safe and spec a shorter head tube. That means guys like me and Jack Moir end up running tall bars and a grip of stem spacers.
I know nothing about Motos, so I have nothing to contribute to that side of the discussion.