Scizor on Gym Defenders tier list

Yeah, he seems like a solid defender with Bullet Punch / X-Scissor

I guess low stamina and not so great moves for defending ?

I am with you that I feel like it should be a solid defender but that double weakness to Fire is pretty massive with only 140 Stamina. Flareon, Charizard, and Arcanine would cook it pretty quick

Fire is double super effective, fire types also resist bug/steel attacks (meaning every single attack either has a resistance cancelling it's STAB bonus, or is neutral), and it doesn't have great defensive stats.

Why would it be on the Gym Defenders Tier List?

Well if you compare Scizor with the other Pokémon who have 4x weaknesses on GamePress' Defender tier list, you'll notice that Scizor's max cp is only in the 2801, whereas Tyranitar, Dragonite, Rhydon and Gyarados all can out-CP a Blissey.

Another thing is indeed that 1.64* weakness to fire, while Fire as an attacking types struggles with a lot of major threats, as long as it's doing neutral or better dmg (eg Ember + Overheat flareon does a crap ton of dmg)

Plus Scizor still takes neutral dmg from quite a lot of common attacking types like Rock, Ground, Water, Electric, Fighting and to a lesser extent Flying (Dnite's Hurricane)

Lastly if we compare Scizor's defensive stats to Steelix then you'll notice the disparity
Scizor: Def 191, Sta 140
Steelix: Def 333, Sta 150

Scizor is an attacker that has a good defensive typing

Because it's weak to fire x1.56

He is an attacker in this meta...not a defender.

Even a poorly built Flareon will bring him down in no time before he can use charge bar moves, or his hp is already ultra low when he could. I tried that on my scizor.