I think it's an issue of sub-sections in the community; I'd say it's a mistake to consider a post, even with 100+ upvotes and hundreds of comments, representative of the community as a whole. Like you brought up, Lee Sin (and if I may add Yasuo, Fizz, Akali, old Kassadin) are melee champions that consistently get threads asking for nerfs; I don't think these are the same people creating threads about Riot's alleged ranged favoritism. (Indeed you get a lot of threads saying that Riot hates ranged champions since they gave Yasuo and Braum projectile shields)
If I were to assume, I'd say the reasoning is somewhat rooted in the mobility creep, which Rioters and players alike have agreed is an issue. More and more champions are gaining mobility, and more dated champions with little to no mobility are having more difficulty keeping up. With the mobility creep, however, ranged champions tend to adapt better than (or at least aren't hit as hard as) immobile melee champions. Ashe and Olaf both have little to no mobility, but Ashe has a bit of an easier time dealing with opponents because she has long range. Both have difficulties (and often receive threads or notice asking for buffs), but Ashe naturally has an easier time than Olaf since it's more difficult to kite ranged champions than less mobile melee champions (which should, imo, always be the case; ranged champions should have an advantage at range, melee champions should have an advantage in close range)
I wanted to make a minor correction and say that melee carries do exist in LoL; Yasuo, Zed, and Kha'Zix are consistently strong picks that see a lot of competitive play, outside of this are champions like Akali, Fizz, Riven, Master Yi, and Fiora, all of whom are very powerful and can single-handedly take over a game, especially when ahead (arguably as much as or moreso than many ranged carries).
I think too that this is partially a phase brought about by the current meta; ADCs are very strong right now, moreso mobile ADCs who have good ability to kite immobile melee champions (we just went through the age of Lucian and Tristana, two of the most mobile ADCs in the game, and while Tristana got nerfed pretty hard, Lucian is still very potent). Getting a super-mobile ADC like Kalista is icing on the cake, and probably triggers some bad memories of pre-nerf Tristana/Lucian where they could put out monstrous amounts of damage while staying out of melee range due to their mobility.
From my limited perspective, the meta seems to cycle like this; earlier in the year, ADCs weren't as powerful and more utility ADCs were favored like Sivir (I remember Sneaky playing Ashe as well), where they have tools to allow melee champions to engage on the enemy team (Olaf was very potent at this time since Sivir's ult gave him the ability to get onto the enemy team). I wasn't on Boards at the time but I doubt people were complaining about Olaf getting kited when he was able to run through a team with Sivir's help.
EDIT: Another contributing factor is how potent Ryze and some other ranged top laners are, which forces a lot of melee champions out of top lane (where ranged champions are brought in to deal with Ryze instead). Again this is simply a meta shift, brought on largely because a lot of ranged champions were really strong in top lane for a long time (ie. Ryze, Nidalee, Gnar). I don't think this was intentional; I can't really say about Gnar, but Nidalee was mostly played in mid lane as a poke mage, where Riot reworked her kit to force her to take more risks in melee form; this in turn (and her ridiculous AD ratio on Q) pushed her into top lane where she could again be safe while bullying the crap out of her opponent. Ryze is somewhat similar, I think.