What is the main reason why Poseidon, Percy's father, left him?

First of all, it wasn't a cow, it was the Minotaur. And the main reason why Posideon couldn't see Percy was that, well, in the book series, Zeus, Posideon and Hades made an oath and swore by the River Styx after World War II that they would not have anymore Demigod children. But Posideon broke the oath when he met Sally Jackson, and so they had Percy. But Posideon couldn't see him because of the oath and that Gods can't help their Demigod kids. That's how it is in the book series, idk about the movie tho! I hope it helped ;)

the gods aren't aloud to see there demigod children and it wasn't a cow it was a minotaur

Once Percys generation was born, Zeus passed a law forbidding the Gods and Goddesses from seeing thier children. Due to a prophecy called the Great Prophecy. It is as follows...
A Half-Blood of the eldest gods
shall reach sixteen against all odds
And see the world in endless sleep
The hero's soul, cursed blade shall reap
A single choice shall end his days
Olympus to preserve or raze

Zeus decided that there can not be any contact because they are not sure which demigod the Prophecy is talking about. I'm not going to give away anything though. Read the books to find out :)

Poseidon left because he was losing his Olympian immortality spending so much time on Earth.(I love Percy Jackson books.)

In the books it was because Zeus had passed a law after or near World War II saying that the Big Three (Zeus, Poseidon and Hades) couldn't have anymore children as they were too powerful. Poseidon couldn't stay as he might risk putting Percy in more danger than he already was and because gods can't have much interaction with demigods.

In the movie, however, it was because as Poseidon spent time with Percy and Sally, he started to forget his duties and turn mortal.