Because as i see it,manaless champions have no drawback whatsoever compared to,everyone else who has a blue bar to abide to.Skills from manaless champions have the same ratios and usually the same cooldowns as those from champions who get restricted by a manabar.So what's the point of playing a blue bar slave,really?
Honestly, Mana probably just isn't in a great state at the moment. At a recent meeting, we were talking about some mage champions and how we felt about their Mana usage, and one of the things we got to talking about was that it would be nice to feel like there were greater benefits to being a Mana champion, because Mana serves almost exclusively as a tool to restrict champions in the early game.
It would be really cool if there was a world where people could feel good about their champion being a Mana user, because it provided them with really cool itemization and playstyle options, or maybe it had really powerful late game implications. Unfortunately, it's just mostly used as a tool to hold harrass and sustain patterns back in the early game, and there are probably opportunities for us to utilize it better.
Please show me a resourceless champion with Xerath level poke. Or even an energy champion -- Zed and Kennen are notorious for the amount of poke they do have without a manabar, but that's nowhere near the level of even a mediocre mage.
Almost all of the truly resourceless champions are melee, and often squishy. Gnar is one or the other.
Now if you're going to say "then what's the point of playing a squishy melee with a manabar", 
think you have a point because they really are a bit weak right now, but most of them have unique things they can do and/or their mana problems aren't as bad as a mage anyway.
What about Energy champions? They never need to get Energy Regen from items.
Edit: Thank you to all the players that are willing to explain the strengths and weaknesses to Energy champions to me! This will help me better understand their strengths and weaknesses. I only own two of the energy users 
& 
. But this will help so much!
Because as i see it,manaless champions have no drawback whatsoever compared to,everyone else who has a blue bar to abide to.Skills from manaless champions have the same ratios and usually the same cooldowns as those from champions who get restricted by a manabar.So what's the point of playing a blue bar slave,really?
ya, there really is no pay off. riot always said "we feel mana champs should be weak when OOM, so this is why we are blah blah blah eve W nerf blah blah blah lux E 130 mana" meanwhile 
E E E E E E E E E E E E E E E E EE E E E E E E Q Q R in 1.5 seconds 
goes OOM just trying to proc his shield "lol, good thing you went OOM, you almost took 1% of my HP"
so stupid to watch this guy dash around minions endlessly with no punishment as a mana hungry champ
The trdeoff is not seeing that lovely color blue.
Its due to the fact that any mana user once they get past 10 minutes, mana become worthless, I have played Lux plenty of times which is one of the most mana hungrey champs in the game and once like 10 minutes go by and she has her Unholy, mana might as well not exist anymore.
But I would say mostly for diversity. It would get pretty boring not to mention wouldn't make sense on a lot of champs if everyone just had a mana bar instead of nothing/energy/mana/hp. Why would Garen have to spend mana to do an overhead swing at you? Why would Lee sin use mana when all his power is from his body? He is using his own energy to do what he does and gets tired if he doesn't pace himself.
Sure you can make the arguement of Riven/Garen/Katarina(3 biggest no resource champs) can use energy, but it really wouldn't make much sense for the most part. Its just an overhead swing, or throwing a dagger, that isn't that exerting vs pushing your body to the limit and flying jump kicking you(Albiet unrealistically but still)
Basically its for more mundane attacks vs the energy ones which are usually a bit more flashy.
It used to be they had high cooldowns and no hard CC 
(cept her ult) 
But then Riot Kept going 
