I assume you're referring to Incarnate Landorus rather than Landorus-Therian.
It's because the boost it gets from Sheer Force is better suited for its special moveset, rather than a physical one. Also sometimes, you actually want the secondary effect of a move, which Sheer Force negates.
Physical moves of Landorus benefiting from Sheer Force
Bulldoze (outclassed by Earth Power, and its secondary effect is generally useful because of its low power)
Iron Tail (low accuracy, poor type coverage)
Rock Slide
Rock Smash (low power, outclassed by Focus Blast, secondary effect is the only reason you'd use this move)
Rock Tomb (low power, secondary effect is very useful, only goes well with Technician)
Secret Power (low power, outclassed by Return, no STAB)
Rock Slide is the only good move that will benefit from Sheer Force. All other strong physical moves - Earthquake, Hammer Arm, Stone Edge, Super Power, Brick Break - do not benefit from Sheer Force.
Special moves of Landorus benefiting from Sheer Force
Earth Power
Extrasensory
Focus Blast
Mud Shot
Psychic
Sludge Bomb
Sludge Wave
Snore
You've already got 4 good moves that will benefit from Sheer Force, and losing the secondary effect of all of these moves is a good compromise for the 30% boost. You will also take no Life Orb damage, and if you pick 3 of these moves + use the last slot for Calm Mind, you can easily sweep an opposing team with the amount of coverage Landorus has between just 3 moves.
In regards to Landorus' other ability, Sand Force has little value when Sand only lasts 5 (8 max) turns in Gen VI.
