Because I, for one, didn't know it was happening and therefore could not participate. My chance of having my opinion count was zero. So pretty much by definition the poll is not representative of the universe of people-who-include-me. And by induction of all the other people who had no chance to participate.

If you want to know more, I recipient looking up the 1936 Literary Digest poll for US President, one of the most famous screwups caused by non-random sampling: https://www.math.upenn.edu/~deturck/m170/wk4/lecture/case1.h...