How did Crocodile become a warlord? Crocodile doesn't even know the basics of Haki and was defeated by Luffy. How did such a weakling became a warlord?
The exact nature of how crocodile was selected to be a Warlord is not known.

However this is what Oda has to say about it


  Similar to Luffy, back when he was young, Crocodile's name spread across the seas with incredible momentum, but soon after he was admitted into the Shichibukai in his mid-20's, he tried to fight Whitebeard and was completely and utterly crushed by him.Source: SBS Vol.78


This seems like a rant question to me. So addressing those issues.

Crocodile was one of the strongest Warlords. He had a powerful Logia fruit with great control over its abilities. He could use its effects over large areas of land. His strength is compared to Jinbei and Ace in the Impel Down Arc.

Bartholew Kuma, Gecko Moriah, Jinbei may or may not have Haki.

Crocodile was one of the most important villains in the One Piece Universe. He beat the protagonist not once but twice. Had an interesting power, introduced us to the Shichibukai and created a very vast organization almost taking over a country single handed. 

Also, I am not sure how you know Crocodile doesn't have Haki? He considered himself strong enough to attack Whitebeard head on. We simply don't see him have any need of it to use anytime he has been onscreen.

Moving out of the world,
Oda had thought of Haki before the series started, but hadn't used it before the Sky Island Arc (Mantra as observation haki). We have at least one clear example of this. Shanks scaring of the sea king. Mihawk may have used Observation Haki to beat the shit out of Zoro on Baratie too.

These may be the reasons that he had left them for later,


He was still fleshing out the idea about Haki. The three colors, there interactions etc. etc.
He saved them for the time skip region to show a clear way that our protagonist's could have a major power up. Ussop, Zoro and Sanji after Haki are a thing to be feared as learnt by Doflamingo.