Nicola was more likeable than Humza (people do vote for personality rather than policy unfortunately)
The SNP is looking like a divided party (Nicola herded the cats pretty successfully) and some people don't vote for divided parties
The next election is (likely) a Westminster one and people will vote Labour (in marginal SNP/Labour marginals) to ditch the Tories
Nicola's competence was questionable, Humza's incompetence is unquestionable
There's probably more but that will do for starters