I guess Riot decided that 47-48% win ratio on a 3% pick ratio is "balanced" enough for the monkey king mains.
Idk, he has an almost 53% winrate mid (1.34% pickrate, almost 20% role distribution), and a 51.7% winrate top (1.72% pickrate, ~25% role distribution). Sure, he's in the 48-49% range for jungle, but his winrate in all three lanes matters. And he was outright broken until recently (Lowest winrate was about 51% in the jungle, was above 55% in top and mid in 8.13, dropped to the 52-53% range for those two lanes in 8.14, and was above 50% bot as well). Not getting a buff is fine, given that he was absurdly strong for over a month.
Meanwhile, Lee sin's current jungle winrate is 48.52%, with a 20% pickrate (op.gg says his pickrate has never been below 8% for this entire season, though, and Lee has always been absurdly popular even when absolutely trash) and 100% role distribution (meaning he's picked in other lanes less than .05% of the time, because otherwise op.gg would end up rounding to 99.9% instead of 100%). His winrate before buffs was only 48.07%, which is only .11% higher than Wukong's current winrate.
And that's just their numbers. Wukong clearly needed nerfs (and hefty ones two patches ago), and Lee could have used some buffs. This is fair and balanced, as Wukong was too strong and Lee was a bit too week. We're not even getting into play experience.
Looking at that, for Wukong, all you have is a statcheck. You have a point and click dash, an AA enhancer, an instant AoE knockup, and instant invisibility. There is nothing you can do against that kit except either just not be there at all, or to slam a stun into his face while he's invisible.
With Lee, you can dodge his Q at the very least (which stops him from dashing in with the other part of his Q), and with the removal of Tracker's knife and the increase of ward cooldowns, it's become far more detrimental for him to try to ward-hop to you - he either needs to blow 75g on a throwaway control ward (and give you up to 30g when you come back to destroy it), or not have vision of vital parts of his jungle for a while (over 2 minutes in early game!) This all works together to make Lee a bit less bullshit to fight than Wukong.
So the difference between Lee and Wukong is not favoritism, but rather Wukong having been broken for most of two months (and still too strong in some lanes) while Lee was (and still is) a bit weak, and with Wukong just being far more bullshit for people than Lee is. Everybody's line of thought when they see a Lee is "ugh, great, another fucking Lee Sin. 0 fucking jungle diversity", not the "fuck this broken-ass autowin cheesy as fuck 0-counterplay monkey and his undodgeable burst" that everyone gets when they see Wukong.
gets on compensation buffs because of an item 
only, which sends a clear message from the balance/design teams that this is as viable as this champ can get and they're not interested on working on him.
while 
gets his yearly LCS compensation buffs.. and what I mean by that , is no matter how good Lee sin is, he'll get buffs just to be seen in pro-play.
I guess Riot decided that 47-48% win ratio on a 3% pick ratio is "balanced" enough for the monkey king mains.
