* Sheer chance. Just because a penny comes up heads umpteen zillion times in a row doesn't mean it has to come up tails the next time. Chance is extremely unlikely, but it is an absolutely valid possibility.
* When matter and antimatter collide, they tend to explode. We might've started out with a lot more matter and most of it blew itself up, leaving a tiny fraction of whichever type was slightly more prevalent. Once the scale is tipped it tends to stay that way.
* They also don't like to live together, which tends to create mirror-twin universes, one of matter and one of antimatter. Imagine if people lived on a magnet, they'd wonder why it was all North because they didn't know about the South end. This and the previous one are moderately possible but difficult to prove.
* Or there could be a mistake in the math. That's quite possible, and straightforward to prove if enough other brains go over it.
* Or what's there is valid but there's a piece missing that would explain the selection toward matter. This is likely. Some things set randomly, but once set, they tend to stay put throughout. Sugars do it too.