why is leblanc a weak assassin
LeBlanc isn't a weak assassin, she's arguably the best assassin in the game. Very speedy, very efficient in the assassination department, reasonably tricky and surprising.
Her issue is she's more a weak champion and a terrible carry.
Most damage rolls can take out a solid 50% of an enemy team's collective HP in a solid team-fight.
A typical mage can take out a solid 50% with their QWER burst's AoE alone.
Any marksman is capable of taking out a solid 50% with decent positioning and auto-attacks.
A typical assassin scales with AD and has solid auto-attacks to fallow up on their QWER, and often resets or a smattering of AoE to hit a 50% goal.
A typical fighter often has a 50/50 reliance on abilities and auto-attacks, single-target and AoE, burst and sustained DPS, and can generally hit close to 50% if they survive long enough.
In all 4 cases you have to do better than average to hit that 50% mark, but it's not exactly uncommon to hit that by accident once in a game on any such rolls.
LeBlanc though? She's pretty much hard-capped at 20% of an enemy team's collective HP in a straight team-fight no matter how skilled or how good her player is. Maybe she can take out 25~30% if a fight really drags on, but that's it.
Which simply means she needs a meta that can circumvent team-fights with picks, assassinations, skirmishes, deep wards, and game-plans that can flat out avoid large 5v5 clashes.
LeBlanc's viability is tied almost solely to how easy it is to prevent a 5v5 from happening. If that's only something a team can reliably do 40% of the time, LeBlanc is going to have roughly a 40% win-rate regardless of how well any individual can do with her.