Think about how many Hashira there are during the main arc and then the ones we know about from the past. 

How many formal hashira in Demon Slayer before Kyojuro Rengoku 

Because he was born with a powerful spirit inside of him

Because he has the spirit of Turbo Granny in him

He's a monster and only turns human to trick people

He was nearly killed and awoke an ancient power

The doctors accidentally used ghoul parts in his operation

She was saved by her brother who promised to find a cure

She convinced the hashira that she would tell them all she knew about Muzan