How did the crucifixion come to be? Is it the pain and suffering on the cross that was the main consideration to the crucifier and that is why that they did it? Or is there some other symbolic meaning for crucifixion?

How did the crucifixion come to be?
It was for a slow, painful and also humiliation. The cross had no symbolic meaning in a crucifixion, it could have been any wooden construction. Historically crucifixion actually means impaling on a plank, piece of wood ,etc. The word cross actually comes from the word crux which defers to a wooden construction(or tree) used for impaling people on. Where it came from we don't know, the oldest registered crucifixion is before ancient Rome by the Persians somewhere 500 B.C.