Christianity is the main religion in both Russia and the United States, so why don't the two unite in the name of Jesus Christ?

Great question!!

Unfortunately, religions are also human institutions, and human failings are too often raised above the virtues of faith, hope and love.

I can't believe that you're actually asking this question

Because America celebrates the perverted version of Christianity. The powers that be would never let the 2 unite to many principalities at play in the US.
I can’t speak for Russia but I bet it’s the same dynamic.

There's plenty of white "Christians" in the US that support Putin.

Maybe it's a white supremacy thing?

Good idea! The Russians incorporate Ukraine and the US gets Greenland.
Hey, that's called world peace, right?

Good question.
American hegemony and Christianity can't possibly mix well. But nobody's going to tell the Americans anything.

Why are there Catholics, Protestants, Orthodox, and lots of other types of "Christians"?

catholic is christian. others are meh

Because when it come to money and power, it ain't the Bible they are reading.