Thats because it's a sniper. Beyond that, it's one of the most powerful sniper rifles in the world.
Snipers are always heavier because they are bolt action, thus having more moving parts. Not to mention it would have to be made of some pretty heavy stuff to keep it from falling apart. The barrel alone must make up for half the weight.
Regular rifles don't produce anywhere near the same bullet velocity of a typical sniper rifle, and beyond that most of then are automatic. I'm not an expert on the subject, but then anything as freaking big as the above rifle would have to weigh a ton.
And besides, snipers rarely need much freedom of movement, since they spend most of their time sitting in one spot for days on end. Infantry need something that they can carry all day long along with all their other gear without getting seriously fatigued (Or heatstroke, for that matter).
You see, I thought pistols weighed like 5-8 lbs, submachine guns weighed 10-15 lbs, shotguns weighed 15-20lbs, assault rifles - 20 to 25 lbs, sniper rifles weighed 20-30lbs, then those big machine guns...about 40-50lbs.
I was pretty off with assault rifles. D: