One possible reason: Game Theory.
Throughout the history of Major League Baseball, there have been 'no-doubt' Hall of Famers during their playing career. This list includes, but is not limited to, Tom Seaver, Ken Griffey Jr., Ted Williams, Nolan Ryan, Bob Gibson, Cal Ripken Jr., Ty Cobb, and Hank Aaron. All of these players are widely regarded as some of the top few that have ever played the game, yet none have been voted in by 100% of the voting populace (Baseball Writers' Association of America, BBWAA). While there are quite a few reasons that a voter may not vote for a player, mostly personal bias, the voters never had to explain why they didn't vote for a player because they could remain anonymous throughout the entire election process.
Voters are allowed to vote for up to 10 players, but can vote for none if they so desire. Beginning with the 2018 election, the BBWAA elected to make all ballots public a week following the results were released. Because of this, many voters already release their ballots within their columns in newspapers, online blogs, or through social media accounts, leading to arguments from fans and other voters. In another recent change to the rules, players are only eligible for voting for 10 years, meaning that if they do not get voted in by their tenth eligible year, they are no longer eligible for traditional Hall of Fame voting. Because of all of these restrictions, as well as the now public nature of the ballot, some voters engage in discourse with baseball fans and try to offer explanations as to why someone did or did not get certain votes. Which leads to the 2016 election...
Ken Griffey Jr. received 99.3% of the vote and was a player whom many fans and writers alike feel is the greatest all around baseball player of all time, who may have broken even more records had he not been burdened with knee injuries throughout the prime of his career. He missed a unanimous selection by only three votes, but has the highest percentage in the history of the Hall of Fame. Despite that, fans were more angered with the three voters who didn't vote for Griffey Jr. than they were excited for him to receive the highest voting percentage of all time. And of course, writers scrambled to find answers for those fans. One writer for the Pittsburgh Pirates, Tom Singer, brought up the idea that some voters may have chosen to forgo Griffey Jr. to give votes to other deserving players who would likely be overlooked in that ballot. Because Griffey would be a no-doubt selection in his first year of eligibility, some voters might decide that he didn't need their vote and should instead vote for a fringe player who might be knocked off the ballot. This idea is simple game theory, that may have been used for decades of voters, but because of social media and the ability to publicly decry ballots, that reasoning has brought itself to the forefront of voting discussions.
Now in 2019, Mariano Rivera is slated to become another first ballot Hall of Famer, with many fans believing he should be the first unanimous selection because of his status as the greatest closer of all time. As of 1/10/2019, Rivera has 100% of the 164 public ballots currently cast (via @NotMrTibbs on Twitter, bit.ly/Hall19 for spreadsheet). One voter has already decided not to vote at all this year because he does not believe closers should be in the Hall of Fame, but did not want to get in the way of history (if a voter does not send in a ballot, that ballot is nullified and does not count towards the totals). Still, I wouldn't be surprised if Rivera does not end up unanimous and those votes are given to fringe players such as Fred McGriff or Scott Rolen, both strong candidates but lack the 'star factor' that would make them an automatic candidate.