This comes from the formation of modern democracy. There were parliments all over Europe with lots of smaller parties or even independent politicians. Parties that would agree with each other would often sit together so you often ended up with right and left wings of these parliments. As it happened they split on the great political debate on the distribution of power between the newly formed labor class and capitalist class. And in most of the parliments the labor friendly politicians sat on the left while the capitalist friendly politicians sat on the right. Obviously each parliment was different and political divides were different but this was the general trend. So the left-right divide became a common politican term.