assassins lack much counterplay like adcs
most champs are actually fighters are something close and much of the game is balanced around such balanced champs however the game tends to be decided more by champs that have less stats and more specific steroids like assassins and adcs
However, unlike adcs its really obvious when an assassin has just absolutely destroyed you and this makes players cry moar
edit
Important note
Part of the reason assassins are so obscenely strong in low ELO is because they are typically countered by support, but people in low elo don't appreciate the support role or know the first thing about it really. They think support is just a ward bot that doesn't get to play usually but then can't figure out why assassins keep shitting on them