Laning against Non-Resource/Energy Champs is Oppressive.

Mid laning against a Lux when she is using her E to harass is something that most players hate, the only upside to laning against her is that at some point, you know she'll have to stop since she has a mana cost. And because Mana regen is not quick, you know its going to be some time until she'll think about using it again.

Mid laning against a Katarina or a Zed however, is frustrating and - key word - Oppressive. Because these characters are not gated by mana costs and Energy regens at a much MUCH faster rate, its easy to see that no matter how many times they fail to hit you with an ability, they are sure to do it again. And again, and AGAIN. And that is Oppressive.

Playing characters who use mana as a resource and going against a Zed/Kat/Yas is a one sided laning phase that isn't centered around making "plays" or well executed strategies. It more or less becomes an unfair dodge ball game.

Its definitely not just the fact that Mana is becoming a hindrance, but the fact that Mana is not a hindrance to those champs is what makes mid laning much less appealing as a standard AP Caster. Building to have a good pool of sustainable mana puts damage on hold and vice versa. You'll stick around more but less damage or more damage, but you'll be useless after a rotation or two.

Resourceless champs get to cut right to the chase. Damage of their choice and, if they are able to use it well, then Life Steal or Spell Vamp. The items that offer both stats are particularly cheap for what they offer, and they do a good job at making harass seem pointless while laning.

Tl;dr - Champs with no costs have less necessary stats needed to build. Mana overall is problematic now which leads to Oppressive Mid Laning.