Could you briefly say why that is?
Briefly ... Jews are strict monotheists.
So is Christianity. Are you saying belief in the Trinity is not monotheistic?
In my opinion, and the teachings of Judaism, it's not.
The Trinity just speaks to God's personage, God is relational..As we are mind, body and spirit..and these three parts work perfectly with one another..that is how God works. Three parts of one Godhead. It's still one God. Even in Genesis it says:
'
Genesis 1:26-28King James Version (KJV)
26 And God said, Let
us make man in
our image, after
our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth."
The Trinity was spoken of from the beginning.