There is no need for "why" after "reason". Because your opening is in the present tense, I would use "is".
My favorite soccer team just lost in the playoffs.
I say to my friend:
One of the reasons why I'm coming home early is/was because I wanted to see them in the next round. Now all is lost.
Are both okay in the sentence?
But was wouldn't be wrong?
but it was one of the reasons why and now that the team has lost it's no longer the reason. That's why I chose the past tense.
In the past tense, I would start with "One of the reasons I came home early was ...". I see no need for the continuous.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.