Assassins are anti carries and...
and supports are anti anti carries. That, I believe, is the core reason neither assassins nor supports are as appreciated as carries or other classes.
Assassins "carry" by preventing the enemy team's carry from carrying. Supports "carry" by preventing the enemy team's assassin/carry killer from preventing the carry from carrying.
Confusing stuff, I know. But my point is that the lack of appreciation towards these classes are because they are all balanced and based off of carries well, carrying. They themselves don't win games unless they are really fed.