I think this was just one issue of many that led to her loss, but to answer your question, it’s somewhat simple:

There are 2 viable options. Voting for one shows support for their platform. But if the 2 options stand together on an important issue and the voter is on the other side of it, the voter cannot support their stance by voting for either option. Voting for either would only serve to erase their opinion which is counter-productive.

If a voter’s opinions cannot be expressed by voting for either option, then it leaves one final recourse to be heard: To not vote for either of them.

This shows up as a drop in turnout. A substantially poor turnout means that there are voters that can be picked up next cycle if either party cares to cater to them. If any parties do this, then the non-voters have successfully exerted influence.

Maybe you disagree that this is a logical strategy but consider this:

Some citizens tried this and lo and behold, their voices were heard. The whole internet is up in arms! All that remains now is to see whether the politicians listen.

They probably won’t, but we wouldn’t even be having this discussion if people turned out to vote against their beliefs on the matter. Which means that, on this issue, the strategy of withholding votes has already been more successful than any outcome that could be arrived at through voting (because, again, those outcomes would only serve to silence these potential voters).