I agree with the marketing comment and reason. Truth is that 99+% of people that buy bikes would not see a realistic and significant difference. So, we all end up buying what looks "pretty or cool" to us regardless of terrain ridden. Sure, you might shave a few seconds here and there in a huge mountain with a super light bike or "feel" a bit faster in your competitive group rides, but in the end we all just "weekend warriors". Buy the one that appeals to you and enjoy it!!!
If you've been in WW forum for a while, youd have noticed trends. Thisis a focused group. When I first joined, there was an insane amount of focus on lightest parts, tuning stuff, filing stuff, drilling things, replacing bolts, stripping frames and everything, etc. Almost every thread was about dropping weight or showing how you did it. Over time, as a lot of the parts got lighter by themselves, we hardly see that anymore. If you look at the top 40 threads right now, I doubt 4 have that focus. Seems everyone has turned more into getting a "cool looking new bike off the shelf" and just slapping a couple of lighter parts off the shelve. Everyone seems quite happy to have a 6 or 7 Kg bike when years ago those threads would not even get a reply! It's novelty I think, and you can't show off to others about what you have if they can't tell what it is. Personally, I still think there is a whole new crowd cycling these days, and the aero and posseur mentality is stronger than it used to be. All of this is based on solely my observation of "weekend warrior" as the PRO are controlled by too mnay variables like sponsors, budgets, UCI rules, etc that none of us have to deal with.
Ride and enjoy it!!