The Republican National Convention in Cleveland is giving delegates and viewers even more drama than expected. Melania Trump's plagiarism scandal aside, Wednesday night was a riot — the lights flickered and the Jumbotron glitched as Texas Sen. Ted Cruz gave a rousing speech in which he pointedly did not endorse the party's nominee, Donald Trump. In fact, Cruz only mentioned Trump's name once, and he left the stage to a chorus of boos.

"Don't stay home in November," Cruz said, according to CNN. "Stand and speak and vote your conscience."

Whether you love him or hate him, Cruz's decision to not throw his full support behind Trump was a gutsy one. It was also likely a choice he didn't make lightly — especially because it meant breaking his March promise to back the eventual nominee. Here are five possible reasons the senator didn't endorse his once-rival.

Because He's a Sore Loser 

Cruz came in second in the Republican primary, earning 559 delegates to Trump's eventual 1,543. Cruz promoted himself as a Washington outsider, but many considered him to be the more experienced, established alternative to the billionaire. Cruz ended up with 7.6 million votes and only reluctantly dropped out in early May. 

Because Trump Has Insulted Him — A Lot

There's probably still some Taylor Swift-Katy Perry-esque bad blood between the two politicians. During the primary, Trump repeatedly laid into Cruz, nicknaming him "Lyin' Ted" and flatly describing him as "a nasty guy." Trump also famously insulted Cruz's wife, Heidi, in March when he threatened to "spill the beans" on her. And who can forget when Trump implied Cruz's dad helped assassinate former President John F. Kennedy?

Because Their Beliefs Differ

Many voters think Cruz is more conservative than Trump. The senator has also said he disagrees with Trump's proposal to stop Muslim immigration and wants to cut the tax code, CBS News reported. InsideGov points out that Cruz is tougher on same-sex marriage than Trump and wants to expand free trade. However, it's worth noting that both politicians really hate presumptive Democratic nominee Hillary Clinton.

Because He Wants to Go Against the Grain

Cruz has said before that he's not bothered that many high-up Republicans don't like him, the Washington Post reported. This includes Trump, who he reportedly warned earlier in the week that his speech wasn't going to be an endorsement. Politico even suggested maybe it's kind of his thing — "Cruz showed again that he has no compunction about being the lonely, unpopular naysayer," it wrote.

Because He Still Wants to be President

Of course, the prevailing theory on why Cruz didn't endorse Trump is that he's setting himself up for a run in 2020. The National Review wrote that "there is no question Cruz will run" again, and he could stand to do better with a less-crowded field of candidates. The best evidence of this may be an apparent fundraising email he sent out after his speech Wednesday night in which he employed a phrase that seemed to reference the future: "As our cause goes on."