I will say it's mostly due to the amount of effort creating power-saving features in OSX that are not present in other OS's. It's quite interesting all the little things they do to prolong the battery.

I have Fedora 25 with TLP, it operates fine, there is nothing proprietary blocking it (but you do need TLP, install it asap to get any batt life out of Linux) on my macbook and I can almost reach 5 1/2 hours web browsing (Opera with battery saver). On OSX I can easily obtain 9 hours doing similar work. This is all on a 6 year old battery too.

It's not the GUI, power management tools in Linux are somewhat behind the curve. Or so I'm told anyway