Because both his Q and E reveal and so, assuming they are of equal, high, skill level, Lee will remove an integral part of her kit in her shroud.
She still gets the bonus resists and the minor slow from the Shroud but the main purpose of her Shroud is to stall for her energy to return and to a lesser extent her cooldowns.
When trading as Akali you will all-in someone which drains your energy, then drop shroud (further draining your energy) so that they cannot retaliate to the damage you just did (much, some can with AoE or hitting you before shroud but it will still be less). Then during the duration of the shroud once she gets some energy back she can then zone or further trade with them, dipping in and out of stealth minimizing return damage.
By removing this part of her gameplay it cripples her ability to trade. Lee can extend the trade to continue fighting her while in her shroud when she doesn't have the energy to fight back. Lee does good damage with his autoattacks which also restore his energy so he won't run into this problem as much.