A better question is why didn't matter & antimatter just annihilate each other with 100% completion.
That is an excellent question. As @lemouth expressed, this is still a mystery.
Yet, particle physicist are not in front of a white page here. There are some particle physics reactions that do not seem to be perfectly "symmetric", i.e. that appear to favor slightly matter over antimatter. These reactions are under deep study .
I am currently reflecting on the next episode, that should enlighten you (yes a full episode on this question :-)! I will discuss the first instants of the universe, where matter and antimatter were formed.
Then, I will introduce a really important pillar in physics: the CPT theorem, and briefly mention some of the reactions that appear to violate it in favor of matter.
So stay tuned!