Because Baby Yoda is a member of a mysterious species whom we know very little about, except every example previously seen was a Jedi Master, meaning that it's seems more plausible to believe that manipulating the Force is instinctual to them.  Additionally, he's fifty years old and we know nothing of his past.  Rey is a human, and the previous human force sensitive protagonists we've seen actually needed to be guided and trained in the Force to be able to use most of their powers.

It's one thing to be disappointed.  The franchise has been producing turds since the Holiday Special came out.  But the Sequels weren't merely disappointing, they were insulting.

And to give credit where credit is due, I'm not criticizing Rey's ability to beat Kylo.  She was already established as a skilled melee fighter, and you don't necessarily need the Force to beat someone in a swordfight.