Should also probably be mentioned that NH is one of the most educated states in the country by percentage of adults with a college degree (8th highest in the nation).

Trump performs poorly with white voters with college degrees relative to other Republican candidates historically (even compared to his own performance among college-educated white voters in 2016 he lost support in 2020), and it seems like GOP candidates who associate themselves closely with Trump also perform poorly with them: https://www.bloomberg.com/graphics/2020-election-swing-state-biden-trump-coalition/?leadSource=uverify%20wall

Which is a bad combination for a pro-Trump Republican candidate in NH since there are a lot of white college-educated voters in NH. A more moderate Republican like Sununu doesn't suffer from the same issue and can win in NH fairly easily.