Why? If Americans actually did vote for the lesser of two evils as a matter of course, they wouldn’t have picked Bush over Gore and Kerry, and wouldn’t have picked Clinton over Sanders now.
As somehow who generally does endorse voting for lesser evils, I would advise that having dropped that opportunity, people should look at what their particular region is like. Is there much chance Trump might win there, but maybe Clinton could beat him? Then you should probably vote for her. Is there not so much? Then vote for the best person you can – and that’s strategic too, because the less Clinton thinks she can take progressives for granted, the better. Either way you’d want to promote better candidates for next time, so that maybe we can actually get enough votes for a lesser evil than Clinton, as almost happened here.
Maybe there are better strategies out there, but does this one really deserve all the sneers it gets? People talk like voting for the lesser evil is what brought us to this point, but in fact what’s really been happening is anything but – voting for the candidate who seems experienced, or presidential, or good to have a beer with, or unafraid to be racist. Americans opt for the greater evil at least half the time.