The 34-year-old -- a 2-time cancer survivor -- was supposed to wrestle Goldberg for the WWE Universal Championship during the April 4 and 5 event ... but ultimately decided it wasn't worth the risk to his health.

According to ProWrestlingSheet, Reigns told WWE he's immunocompromised from the cancer fight  -- and didn't feel comfortable performing with the looming threat of COVID-19.

But, on Monday, Reigns -- real name Leati "Joe" Anoaʻi -- spoke to his fans on IG, saying there's more to the story.

"You don't know the whole story. All you know is what you think, 'oh well his health' and this and that, but you don't what else is going on in my life."

"You don't know if I have newborns. You don't know if I have family in my household, older family. So, yeah, just like the old saying, 'Don't flip that cover open and read a few pages, get into a few chapters before you really start running your mouth.'"

He continued ... "For all my fans, I'm sorry. I didn't get to compete this year and put on a show and entertain but sometimes things are more important."

"I had to make a choice for me and my family."

Reigns said he traveled to Orlando with the intention to perform -- but he had a change of heart ... explaining, "I had to make a decision for me."

"It doesn't take anything away from what I've done and what I'm going to do. I still love this game. I still love pro wrestling, sports entertainment. I love being in the ring."

Reigns acknowledged that's it's "a little weird" performing in an arena without fans -- but says the bottom line is he's ready to get back to work asap.