Allegedly because Trump voters are less “engaged”, which is a metric that is supposed to correlate negatively with the likelihood of voting. Disengaged voters support X, but when it comes time to vote, they do not bother. I do not know whether any of this is true, but traditionally Republicans’ supporters were MORE engaged, so the “likely voter” shift benefited R’s. Not so this time. Allegedly.
I do understand the concept of being less engaged, and therefore less likely to vote.
I just don’t know why you would think that of Trump supporters - our side is obviously more engaged - not less.