Because, in the U.S., neither Democrats nor Republicans have a plurality of support among voters. Either party being more popular than the other presupposes those are the only options. But the largest "political party" by voter registration is independents, aka, unaffiliated voters. (There is an actual Independent Party, but it's not worth mentioning and has very terrible numbers. To distinguish, if I have to refer to them again, I'll use "Independent Party" and will endeavor to keep "independent voters" lower case). Independent voters number slightly above one-third of all registered U.S. voters, while Democrats tend to be slightly less than one-third. (Who is more popular is a moot question as they're so close it's likely fractions of a percent.)
Independent voters, by dint of the fact that they have no party, are a tough nut to crack in statistics and polling because they aren't an organized party (not in the sense of the classic joke "I do not belong to an organized political party! I'm a Democrat!) so there are no shared plank issues among them... or united ideas. Some are Independent because Neither party is a good fit for their interests... some are independent because they like some issues supported by one party but think another party is better on other issues. Some are more likely to vote for one party than they are for another, but don't want to be stigmatized for affiliating with that party (i.e., a Republican supporter in a community with a large Democratic base may loyally vote Republican but say he is an independent to avoid hostilities when talk of politics comes up). Some think both parties are really the same party that gets nothing done. Others will vote for the best person for the job, party be damned. (I've voted in four elections and by my third election, I had yet to vote for any party's candidate twice.)
That said, polling wise, ignoring the third party and what they tend to want from elected officials is a good way to get yourself a lost election (when the races aren't lopsided for one party over the other). Especially on the presidential level. Having been a registered independent in a swing state, I can tell you that the candidates for president took efforts to make you feel like your vote mattered. (I once told the first live human person for a particular party that if he did not remove me from their bot-call list, I would vote for the other candidate because "Four More Years of Bush is Four More Years of the 'Do Not Call' registry. And before you mention that "bot-calling" is still legal for politicians... oh yeah... I knew that... but every vote counts, and undecided votes count more than the decided ones. To paraphrase Azula, the party affiliated have already decided your fate, I'm still mulling it over.").
As of recent polling, it seems independents tend to take a soft Republican position, but that doesn't mean that they won't vote for something favored by Democrats. And it's also important to note that for presidents, it's not a popularity contest... it's 50 popularity contests.