Because it is a counter lol
The second most common Scizor set in the ~1850 ELO range is Careful with 248 HP/8 Def/252 SpD which is ran 11.008% of the times. With this spread, even Choice Specs Lele does only 37.5 even with Focus Blast, on average, which is a mere 3HKO. Scizor, with its great Steel-/Bug-typing resists both of Tapu Lele's STAB options and takes less damage from its other coverage options.
This doesn't make any sense, as Lele's Psychic terrain doesn't allow Bullet Punch to work, and most of the Scizor's I'm seeing are running Bullet Punch/U-Turn/Roost/Defog or something like that.
Knock Off > Defog is far more common, but if you're running Defog, just Fog the Terrain lmao. Scizor, even with Bullet Punch disabled, is winning the 1v1 even if it comes on a manual switch, as U-Turn does 46 on average, and Knock Off knocks the item and deals a nice amount of damage, so even better.
Plus, due to Scizor's not great SpDef, I guess Lele 2HKO with specific attacks.
Then, why is Scizor treated as a Lele counter?
252 SpA Choice Specs Tapu Lele Psyshock vs. 248 HP / 8 Def Scizor in Psychic Terrain: 126-149 (36.7 - 43.4%) -- guaranteed 3HKO
252 SpA Choice Specs Tapu Lele Moonblast vs. 248 HP / 252+ SpD Scizor: 97-114 (28.2 - 33.2%) -- guaranteed 4HKO
252 SpA Choice Specs Tapu Lele Focus Blast vs. 248 HP / 252+ SpD Scizor: 163-192 (47.5 - 55.9%) -- 80.9% chance to 2HKO (nice probability but Focus Miss, and with the miss chance + 80.9% 2HKO chance (and the SpD drop into the equation too), Scizor is winning usually)
252 SpA Choice Specs Tapu Lele Psychic vs. 248 HP / 252+ SpD Scizor in Psychic Terrain: 119-141 (34.6 - 41.1%) -- guaranteed 3HKO
Other sets (Scarf, Calm Mind, etc) hit like noodles.
So, Scizor is termed as a Lele counter because it is a counter and wins the 1v1 usually even when it's in on a manual. And therefore, Lele is sometimes paired with Magnezone so that it can trap and eliminate Scizor's.
Hope it helped.