Because of the level of intimidation and abuse which is hurled at anyone who dares voice an opinion which the P.C. liberal Gestapo don't agree with and therefore think you cannot have, self-censorship takes place on a massive scale.
It is all designed to close down freedom of speech and opinions on issues which they don't agree with.
We've all heard it; if you don't agree with unlimited immigration you're a racist xenophobic.
Tell a woman how nice she looks and you're sexist.
And it would work.
Except that's all precisely why we have secret voting.
Exactly so that people can freely express their true, personal opinion.
People, understandably fearful of public denunciation to the possible detriment of their career and retribution from the P.C. diversity police and 'equality' fanatics, therefore simply remain quiet and keep their opinions to themselves.
Indeed, more to the point, they specifically tell pollsters exactly what they want to hear; i.e. the apparent 'mainstream' opinion.
Saying one thing in public and then subsequently voting in private a different way, is what led to the term 'shy brexiters' and indeed, what has led to the similar apparent phenomena of 'shy Trumpers'.
In reality it's just intolerance of other people's opinion which as we know (because the diversity brigade love to claim the sole rights to) is bigotry.
It's also bullying and intimidation; both also familiar words that the so called 'tolerant' liberal P.C. brigade lay claim to and continually object to.
Whichever way you look at it, it's not good for democratic debate and freedom of speech.
And neither is it helpful to opinion polls which have now basically been rendered to a form of a self-fulfilling prophecy, for the liberal elite, by the apparent liberal elite.