I think the biggest factors were the money combined with months to run ads unopposed. Sinema was able to cement herself as a Trump-supporting Democrat centrist by running ads all summer - while the republicans fought a mean primary battle. Arizona has a late primary, so there are only 6 weeks between the primary and the start of early voting.
McSally has the personality of a dead fish, but she might have won if she had 10-12 weeks to RUN before the voting started. From what I read, the later the votes, the better McSally did. 75% of Arizona votes early, and McSally lost the election in October. She was winning by November...
All good points.