Huge wall of text incoming:
Long story short Marksmen constantly received buffs ever since Season 5 when Devourer, Fiora rework, and Gangplank rework were released. This lead to patch 5.16 which all caused pressure to buff the Marksmen to out of control. Except Ezreal since he's entirely different from the other Marksmen.
It's a huge amount of small reasons since season 5 that bunched up to this big mistake that Marksmen are out of control right now. The game was great, fun, strategizing, and balanced when Cinderhulk was tweaked just right. Everything with the right skill level was viable, except for champions purposefully unviable like Yorick, Warwick, Poppy, etc.
Then Devourer, Fiora rework, Gangplank rework (GP is balanced now but was out of whack for a while after his rework there) were released. This was the first step in starting the power creep. Than the nail of the coffin infamous patch 5.16 happened. Bruiser were not doing so well so power was given to them to deal with all the Tanks and Marksmen. Juggernauts is a good concept really, immobile melees should be a threatening monster. The problem is that nothing was really solved. Juggernauts can destroy anyone but Marksmen because they have only damage and no utility to deal with them. Seeing how Marksmen are always necessary, anyone whom can't deal with them are useless, hence why immobile melee, and now Assassins are non-existent.
With the Juggernaut update also came binary class power spikes. All Tanky champions became mid game, all Assassins early game, most Mages became mid game only a certain few are late. Marksmen kept their dominance of late game, and now are just powerful at all stages of the game. Fun fact before that rule, only Marksmen had class power spike of owning the late game. However that was half the class problem, the other half is all their main damage is right clicking. That's why there were only 3 viable Marksmen all the time, they were all basically the same. Just go with the 3 with best numbers that counter each other. That's why you see the same champions a lot of times now. Less strategy and more on whom has the best numbers.
Then the Marksmen update happened which buffed them through the roof and caused a massive damage creep. Also gained defensive AD items like Phantom Dancer, etc. For a time there it was basically 5v5 of all Marksmen champions, even supports. So everyone else had to try and catch up and more power spike balance nightmares.
Also with the Marksmen update came Keystones. Thunderlord Decree was the number 1 keystones, however that doesn't ignore the fact that Marksmen have the option of 4 Keystones. While everyone else has 1 maybe some have 2 while some others don't even work well with any.
Then the Assassin update came. Great concept and idea, burst needs to be delayed. People have forgotten the days of LeBlanc comming from nowhere and erasing your squishes before anyone even blinked. Those were the stuff of absolute frustration, and the main reason she was #1 banned during season 5's balanced era. HOWEVER the delayed burst's execution was horrible. Yay that Assassin's burst was delayed, but no other class burst was delayed. Included Mages like Syndra whom can burst from safety, or of course Marksmen never got nerfed with their crit delayed burst.
Meanwhile Makrsmen got their hands on lethality, ruining it for Assassins like they did with Sterakk's Gage, Death's Dance, and such.
Then the final cherry on top, the Vanguard update. Tank items were nerfed, support items were buffed, basically all AD items got 10 damage, crit items price were reduced, and GA became and AD item. Another defensive AD item to add. This update was a Marksmen 2.0 update since all tanky champions are now buying support items to keep their Marksmen overlords Alive.
Edit: One major thing to also note, the Marksmen update also brought CDR on every item. This made Marksmen job 100x easier since everyone can blow their load on the front line, and the Marksmen cleans up with no worries. Before people had to worry about using their abilities too soon on tanky champions because then they would be out of fire power right after. Now that everyone has very short cooldown, no one needs to worry anymore. What took a Marksmen about 15 shots to take down a Tanky champions has been reduced down to about 3 since the front line is crippled. Then the one team collapses because of spammed abilities. Summoner's rift is more or less URF.
looks at them. As a 
player I would never try to 1v1 the enemy 
unless they are severely behind because they just simply beat me for the most part (Rock>Scissors). Which brings us to the problem we have now. Assassins used to just simply destroy ADCs but they have had their damage output slowed which allows ADCs to right click them down and kill them and possibly even life steal the Assassin's damage and live. So in a way it's now Paper>Scissors.
or 
?
but that's like 3 champs outa 20........
can beat a assassin that's equal to her and both play well then i will object!
