Why was it almost impossible for one party to win a majority ?

Why it was almost impossible for one party to win a majority because the candidate(s) were represented by north or south. Most south obviously didn’t vote for the north and vice Versa. Since the majority was 50/50 or very close it was almost impossible for one party to win a majority. (You can copy this i didn’t use a website)

Both countries had to aprove annexation.

Im migration prohibition.