Professional sample surveys use careful random samples, usually by randomly dialing telephone numbers, to come close to an SRS. But the results that a sample survey actually obtains may be strongly biased because:
a). the typical sample size of 1,000 or 1,500 people is too small.
b). many people have unlisted telephone numbers and so are left out.
c). surveys report only what their sponsors want to hear.
d). many people refuse to respond to telephone surveys.
Many people have unlisted telephone numbers and so are left out.
This makes it non-random since not everyone has an equal chance of being included in the sample.