] Is this because a) the Dashwoods, though relatively poor have better family connections than the Bennets, b) Darcy is too concerned about his social status, c) Brandon is not concerned enough about his social status, or d) Darcy is of higher rank than Brandon and therefore must be more picky?

it is mainly (a) and (d), and probably (b). Mr Darcy after all, as Mrs Bennet will remind you, has £10 000 a year, to Colonel Brandon's £2000 a year. He also has superior connections, that is to nobility, and a grand estate, with a house in town. I read S&S a long time ago, so I can't remember how much the Colonel is worth overall, but surely Mr Darcy must have a higher position in society! Isn't he supposed to be the most illustrous hero that JA created...?