1. Money: We knew headed into this race that Booker was a fundraising behemoth. He's lived up to his reputation. Booker raised a whopping $4.5 million during the second quarter, and had nearly as much money in the bank a month out from Election Day. He added another $2 million+ during the first 24 days of July. Only Rep. Frank Pallone (D) has really even come close in the money chase, but even he is a ways off Booker's pace. The reality in a populous state like New Jersey with big media markets (New York, Philadelphia) is that you have to be able to raise money to compete. Booker has been setting the pace, and the opposition simply hasn't been able to match him.
2. He's very well-known. Name recognition, name recognition, name recognition. If people don't know who you are, they aren't going to vote for you. Booker is the best-known pol in New Jersey other than Gov. Chris Christie (R), and he has even carved out a bit of a national profile. In Quinnipiac University's first poll of the Democratic primary, nearly three in four voters had an opinion of Booker, and most were positive. Pallone and Rep. Rush Holt (D), by comparison, clocked in at 30 percent and 31 percent name ID, respectively. "Who are those other guys?" asked Quinnipiac pollster Maurice Carroll at the time. "The record shows that Congressmen Frank Pallone and Rush Holt are big in their districts, but state-wide, no one knows them." Booker had a big head start out of the gate, and he hasn't looked back.
3. Time. Christie set the date of the special election primary June 4, giving candidates barely more than two months to campaign for the Senate. Given the built-in advantages Booker had from the start, his opponents needed time to eat away at his campaign. An open race with a much longer campaign (think now-Sen. Ted Cruz's upset of David Dewhurst in 2010) might have made things more interesting. But given the limited window, time was never on the side of Booker's opponents.
4. He hasn't really gotten hit. Until very late in the race when Holt released a TV ad calling Booker "no progressive," the Newark mayor had largely received a free pass. It wasn't like there wasn't anything for opponents to try to use against him. Booker's record as mayor of Newark has come under scrutiny by, among others, the New York Times. It may be the case that Booker's opponents understood that he had a very good chance of soon joining the Senate, and that if the race never really moved (and it didn't move), it wasn't worth being the pol who went hard after a future senator from the same party. Also consider that Holt and Pallone are not giving up their seats to run, and would be in the same congressional delegation as Booker if he wins. Sure, Holt and Pallone wouldn't be working directly with Booker, but it wouldn't necessarily behoove them to begin that dynamic on the heels of an all-out effort to criticize him.
5. Booker didn't make any mistakes. When a candidate starts a campaign in the position Booker does, it typically takes some big blunders to open the door to an upset. Booker simply hasn't committed any major errors. Recent polls show Booker pulling upwards of half the overall vote in the primary. If he advances as anticipated Tuesday, the general election campaign isn't expected to pose any more of a threat to him than the primary did. New Jersey is a Democratic state, and Republicans were unable to recruit any top contenders to join the race.