We want to know why Biden voters are not shy, and why Trump voters are shy.
Some might call that a shuffle of an answer, but I certainly would never do that. :-)
This is a statistical question. Obviously, the answer for each person will be different (your second answer may be valid for many, but likely not all), and is challengeable (as Lehigh was challenging one reason below). But, we do know that a statistically noticeable portion of Trump voters don't do polls.
I tend to think that the Democrats have been successful at stigmatizing their opponents as haters, not because they are all actually haters, just that there is a stigma which attaches (by the media in public discourse, mostly, and by Democrats themselves in private conversation). And, the "shy voters" are just not revealing themselves in any way that might trigger the stigma being applied to them. Whether this is rational or not for each individual, we all know that statistical effects are felt at the margins. That is, the stigma doesn't affect the truck-flag dudes, or the self-employed, or the simply robust & confident individuals, but it does affect some people in a group (the margins) whether it be only people who are more sensitive, or people who work in more sensitive work environments, or people who are less assured of their employment and don't want to stand out....
It is unhealthy for simple disagreement to be stigmatized in a Republic. We know the Left has tried to silence people who have religious beliefs supporting a heterosexual lifestyle (to pick one example). Should the Left then be surprised when such people don't show up in polls? If they simply engaged those people in political discourse rather than tried to embarrass or harass or shame them into silence, they would not have this polling problem.