FWIW I think the theory is that you have some window before whatever on your teeth turns into hard plaque. So brushing in the morning helps remove any accumulation that happened overnight. I guess brushing after breakfast might be slightly more efficient in the sense that it will clean away/remove food remnants you've just eaten but not sure how much difference it makes in practice as long as you're brushing again later (e.g. before you go to sleep).
EDIT: Technically plaque forms faster but only hardens into "tartar" after about 24 hours or more.
EDIT2: There is another reason to avoid brushing you teeth immediately after a meal. Supposedly they're softer due to higher acidity or something along those lines. It's recommended to wait 30-60 minutes after a meal before brushing.
Re: when to brush… it’s fascinating to me how little fact-based debate or discussion there really seems to be about this type of personal care custom. Most people just do whatever they’ve always done and haven’t had the chance to hear arguments (besides shallow ones from a roommate or spouse that amount to “that’s not how I was taught.” It’s nice to hear actual reasons.
EDIT: Technically plaque forms faster but only hardens into "tartar" after about 24 hours or more.
EDIT2: There is another reason to avoid brushing you teeth immediately after a meal. Supposedly they're softer due to higher acidity or something along those lines. It's recommended to wait 30-60 minutes after a meal before brushing.