Some champions are almost rarely played; such as GalioSquare Galio, AniviaSquare Anivia, etc. They are actually pretty good, but their difficulty is usually high. Here are some reasons why none of them are played often:

AniviaSquare Anivia:

  • The toggle on Glacial Storm Glacial Storm is really hard to manage, draining mana alot, which some players find difficult.
  • She has one of the lowest starting health.
  • Using Crystallize Crystallize incorrectly might lead to your death, or your whole team's death, because during panic it is difficult to place.

CassiopeiaSquare Cassiopeia:

  • Requires difficult and perfect timing in order for Petrifying Gaze Petrifying Gaze to stun.
  • Scary noises during death and Petrifying Gaze Petrifying Gaze.
  • Noxious Blast Noxious Blast is delayed and can be dodged easily; Miasma Miasma can be avoided easily, even with the expansion.
  • Deadly Cadence Deadly Cadence is hard to master for new players.
  • Twin Fang Twin Fang used twice and all other abilities can make the most of Deadly Cadence Deadly Cadence; however, it is very difficult.
  • Not even Mobafire has a troll guide for her.
  • She ugli.

GalioSquare Galio:

  • In order to successfully pull off a good teamfight, the standard combo is flash and to use Idol of Durand Idol of Durand, which is kind of a waste.
  • Very mana hungry.
  • Resolute Smite Resolute Smite and Righteous Gust Righteous Gust requires skilled placement, as mobile champions can easily dodge them due to their slow projectile speed.
  • Accidentally moving while casting Idol of Durand Idol of Durand will mess up the whole channel and ultimate, which means you must be steady and not misclick.

GangplankSquare Gangplank:

  • Cannon Barrage Cannon Barrage can be easily moved out of by mobile champions.
  • Even with low cooldown, Parrrley Parrrley deals small damage and drains mana; also countered easily by YasuoSquare Yasuo with windwall in top, since Parrrley Parrrley is Gangplank's main damage output.
  • Hard decisions on whether you should use Parrrley Parrrley to poke or to last hit and get gold.

MordekaiserSquare Mordekaiser:

  • Some people hate Brazilians. Huehue.
  • Lacks range unlike other popular mages on his basic abilities. Siphon of Destruction Siphon of Destruction can easily miss a champion and waste his health, while Mace of Spades Mace of Spades and Creeping Death Creeping Death requires him to be close range.
  • He calls the summoner a fool.
  • No escape or crowd control.

PoppySquare Poppy:

  • Can be called Poopy.
  • Second lowest mana at level 1, lowest mana at level 18 (see here.)
  • Has been a while since attention has been brought, maybe a rework rito?
  • Can continuously be crowd controlled during Diplomatic Immunity Diplomatic Immunity until it wears off and then bursted down.

RammusSquare Rammus:

  • Unappealing splash art which brings no attention.
  • While chasing or escaping, Powerball Powerball can be interrupted by minions.

RumbleSquare Rumble:

  • Junkyard Titan Junkyard Titan will silence yourself, which is bad for teamfights or trying to escape with Scrap Shield Scrap Shield.
  • An AP champion going top is not really ideal.
  • The Equalizer The Equalizer is a vector skillshot, which is hard to aim with smart cast and is long to aim correctly without smart cast; missing it could waste your whole ultimate.

TaricSquare Taric:

  • Nerfed alot of times.
  • Unlike most supports ultimates, Radiance Radiance is a point-blank self-targeted spell, which requires him to go in, and with no dashes, he has to flash.
  • Gai

UrgotSquare Urgot:

  • Ultimate deals no damage, unlike most adc's.
  • Different playstyle which users are not used to, not your average autoattacker.
  • Mana hungry.
  • Doesn't go straight damage, which users who want to go full damage don't like.
  • Usually considered a "troll" champion.

ViktorSquare Viktor:

  • Death Ray Death Ray is the most damaging basic ability and is best for harassment; however, it is a vector skillshot, and hard to aim.
  • The Hex Core, for most people, is hard to manage, because it takes an inventory slot.
  • Chaos Storm Chaos Storm moves the closer it is to Viktor; which is difficult to long range kill and kill from a far distance in teamfights, unlike other mages who can (KarthusSquare Karthus, LuxSquare Lux, Vel'KozSquare Vel'Koz, etc.)

YorickSquare Yorick:

  • Has never been in a free week, so users who haven't bought him have no experience, practice, or knowledge, making it difficult for new players who bought him.
  • Unholy Covenant Unholy Covenant is useless if you have no mana to summon ghouls.
  • Easily harassed from long range and not able to poke because of no poke if his ghouls are focused on minions.
  • Omen of Death Omen of Death is useless if the enemy(ies) are mobile and can escape.

ZacSquare Zac:

  • Elastic Slingshot Elastic Slingshot can be interrupted during escapes.
  • Bloblets can be easily be bursted down and even smited.
  • During jungling early game, it is key to pick up your shed chunks, which requires walking around the camps, which makes jungling longer.

Will do more later.