Before someone accuses me of being a Darius Main who just wants his champion to get buffed: While I am/used to be a Darius Main, that isn't why I'm writing this. I writing this because it makes no sense for him to have mana. So just let me explain.
1. It doesn't make sense considering the lore.
(If you don't care about the lore you can just skip this point)
Similar to Garen, who doesn't have mana, Darius is just a guy in armour. I realize that this is true for other Champions like Xin Zhao, but most of them have at least some non-physical ability that couldn't be performed in real life. This isn't the case for Darius. Chemist just a guy in armour who swings his axe around. In no way would he require anything but immense physical strength to do so. He's not a mage, doesn't have anything remotely magical, doesn't use technology and isn't a super natural being.
2. It doesn't make sense from a gameplay perspective.
Darius is never out of mana. If you play him a lot you will find that there is only one stage of the game, where Darius is ever oom. And that is if you haven't recalled until level 6. Other than that his mana is always enough. But not having enough mana to use your ult level 6 is terrible. It feels bad, but doesn't really affect a lot of games. It might be relevant in 1 out of 50 games or so, where you miss out on a double kill because you didn't have 100 mana to ult. Meaning 49 out of 50 games Darius having mana doesn't matter anyway.
3. Conclusion: The only reason for Darius to have mana is because he is a champion in league of legends. It doesn't make sense looking at his lore or gameplay and removing it wouldn't have a notable effect on him.
So why remove it then, if it doesn't have a huge effect on him?
Because in 1 out of 50 games it costs you a double kill, which is frustrating as hell and even more aggravating when the only reason is that Darius didn't have enough mana to jump or swing his axe in a circle.
Feel free to give me your thoughts, bring up reasons why he should/shouldn't have mana or insult me for playing Darius (if you I won't take you serious btw).