One huge reason is that a weight on a road bike only mattter accelerating and climbing. Otherwise , according to physics, it doesn't matter at all.
On a mtn bike, you are moving the bike much more than a road bike, the lighter the bike is, the better it responds to your movements.
And yes, on a mtn bike I would definatly sacrifice weight on brakes and forks etc, but also you can save tons of weight were you can.
For reference, my road bike is 18.5lbs ready to ride, no bottles, lowish end with 105, carbon frame., my mtn bike is 19.8, high end spec, carbon everything.
Also, idk if its just me, but it is much easier to gain weight on a mtn bike. 1900gram wheels with a 4.5 lb fork a 7 pound frame, 700gram tires, sh## adds up fast. And none of that stuff is terrible heavy or light.