Andrea Campbell faced off against Michelle Wu, literally the worst matchup.

Wu worked for Warren's 2012 senate race, heading up Warren's outreach group to get votes from the black, LGBT & women communities, particularly in Boston.

Thus, Wu had a strong black outreach a full 3 years before Campbell even ran for city council.

If you wonder why the Boston black community voted overwhelmingly for Wu over Campbell & Janey, it's because Wu has done a lot more outreach with the black community than either Campbell or Janey, for a longer period of time, as Wu started her black outreach campaigning a full 3 years before Campbell ran for city council, a full 5 years before Janey ran for city council & Wu started her outreach under Warren's hugely funded first senate race.

So when Wu ran for mayor, she already had 9 years of black outreach, a larger ground team & had the support of Senator Warren's team too.

You're complaining that there was a mere 14,761 vote difference between Wu & Campbell, but fail to state that Wu had 36,060 votes vs Campbell's 21,299 votes, meaning that Wu had a 69% lead over Campbell. Campbell needed to almost double her votes in order to match Wu. A 14,761 vote lead wasn't a small lead - in the Boston mayor race, it was an enormous lead.