Why was there no room at the Inn?

The inn was more a guest room then an actual hotel like establishment.

There was no room because a census was taking place around the time of Jesus' birth.

People were travelling far and wide to get to their cities of birth, so there was likely someone staying in the guest room already when Mary and Joseph came by.

Changing is right: The word Luke translated as "inn" is the word kataluma, which means "guest chamber" or "upper room". This was at Joseph's family home, but was already full when he & Mary got there, so they were stuck in the ground floor room where the animals were kept. (Yes, they kept the animals inside the house at night.)

The Romans had demanded a census be taken which required citizens to return to their place of birth. Due to this people that no longer lived in the place of their births had to find lodging.

There was a salmon farming convention.