It's not like assassins are weak. Its more about ADCs being worshipped and forced to be excellent kiters to be useful. To catch a Kalista you now need point and click CC/two gapclosers or more/Veigar's E/long range abilities. To catch a Vayne, you need pinks/not be any kind of tank or bruiser because of her anti-melee kit/longer AA range/AoE effects/a pull.
Meanwhile, to catch an older ADC like Ashe, all you need to do is approach her from a location that isn't warded, she won't dash, knock you back or anything, and even if she does stun you or flash, one gapcloser that isn't Volibear/Garen/Udyr level of frustrating will catch her.
The high damage dealers are in a state that they get to burst down defense stackers that are in the game precisely to punish them for focusing the tank. BotRK, LW, VS, lifesteal and all that %HP you see everywhere is another hit to the stomach of the feeble tank race that hardly even manages to stand up now, because engages are becoming more and more pointless, burst is ten times more cost efficient than defense, and every support/ADC gets to completely nulify any attempt at picking off a carry unless they make a mistake so big that the game is already 1-sided by the time teamfights start.
Then we also have the mana regen nerf problems, that make buffing assassins impossible since they already dominate mid. We have ranged champions going top, bullying better than most melees and outscaling them too, we have the insane mobility meta that makes running as a gapcloser completely pointless, we have little viable tank items, low mobility tanks that can't even fit the meta, an overall anti-melee meta enforced even by the new champions that have come out (Kalista/Gnar).