Because Lee impacts the game early. He also puts out far more damage and pressure early without any huge weaknesses. Zac can be pretty much negated so long as you throw a single MR item into your build like Abyssal, Mercurial, or Hexdrinker.
Another big part is that Lee was balanced around having to Q>R>Q people to kill them, in other words landing Q first. In the past two years or so players have realized they can ward hop, Ult, Q>Q with little to no outplay potential. There are no skillshots you can dodge when his damage is supposed to depend on them.
In comparison to Zac leap Lee has much shorter CD's and doesn't rely on it as heavily. Personally I also find it easier to dodge so long as you watch out for the shadow.