Do you even have any idea what you are talking about? There are literally 7 Pokemons that resists its STAB, with only 2 relevant ones in Rotom-W and H while it hits 407 Pokemons Super Effectively with its STAB alone, undoubtedly a good STAB combination, and best in the game.
And guess what, according to
http://stats.pokemon-online.eu/XY OU/473.html, usage for SR on Mamo is 53.50%, not 71% as you fluffed from somewhere, it's still high, but it makes your statement less credible than it already is, and those 53.50% that use it is because Mamoswine does indeed has the slot for the move, but not as a main niche, as you have claimed.
Minus the obvious ones, here is a good list of stuff that Mamoswine simply beats with some of its most commonly used moves.
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Landorus: 374-442 (116.8 - 138.1%) -- guaranteed OHKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Mega Garchomp: 302-364 (84.3 - 101.6%) -- 18.8% chance to OHKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Thundurus: 229-273 (76.3 - 91%) -- guaranteed OHKO after Stealth Rock
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Mega Pinsir: 148-174 (54.4 - 63.9%) -- guaranteed 2HKO
252 Atk Life Orb Mamoswine Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 299-354 (92.2 - 109.2%) -- 50% chance to OHKO
252 Atk Life Orb Mamoswine Earthquake vs. 252 HP / 0 Def Tyranitar: 393-463 (97.2 - 114.6%) -- 81.3% chance to OHKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Multiscale Dragonite: 179-213 (55.2 - 65.7%) -- guaranteed 2HKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Multiscale Broken Dragonite: 359-426 (110.8 - 131.4%) -- guaranteed OHKO
252 Atk Life Orb Mamoswine Icicle Crash vs. 252 HP / 184+ Def Gliscor: 473-562 (133.6 - 158.7%) -- guaranteed OHKO
252 Atk Life Orb Mamoswine Icicle Crash vs. 252 HP / 252+ Def Mandibuzz: 252-299 (59.4 - 70.5%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Mamoswine Icicle Crash vs. 252 HP / 0 Def Togekiss: 377-447 (100.8 - 119.5%) -- guaranteed OHKO
-1 252 Atk Life Orb Mamoswine Icicle Crash vs. 252 HP / 244 Def Landorus-T: 411-489 (107.5 - 128%) -- guaranteed OHKO
-1 252 Atk Life Orb Mamoswine Earthquake vs. 252 HP / 0 Def Mega Mawile: 237-281 (77.9 - 92.4%) -- guaranteed 2HKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Latios: 205-244 (67.8 - 80.7%) -- guaranteed 2HKO
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Latias: 187-221 (61.9 - 73.1%) -- guaranteed 2HKO