Historically, the original cross was an X, as in pictograms from neanderthal drawings and the like. The upright symetrical cross was used also, but as someone pointed out, the X was a universal sign of a crossing, whereas the symetrical + was meant as something else, an early sun sign, that transmuted to the "swastika" design for the sun.

Even the Romans got into the act, when they strung up prisoners they would make the X out of lumber and the prisoner would be splayed out on the X with all four limbs, where they were generally shot full with arrows as a target and then left to die. It was easier to build than the tree Cross depicted on crucifixions, as they didn't have to dig a hole to plant the one main support. Just needed a support in the back. Over time they got the idea of the upright cross that we all know, that would be more cruel and punishing and hence serve their purpose better. And now when we think of cross we mainly see the picture in our mind of the Christian version, and that is more cultural than anything. Another culture would more easily picture the X when the hear the word cross. A little known fact, there, Normie.