Why is that?

If you ate something acidic, brushing might just rub the acid into your teeth. I’m not quite sure how big of a deal it is but that’s my guess as to why you want to wait a bit.

Acidic foods and drinks will temporarily soften the enamel in your teeth. Brushing right afterwards can remove the softened enamel.

Brushing right after drinking acidic soft drinks or soda is meant to be much worse for your teeth than not brushing.

Thanks for this comment. Going to change my eating habits as a result.

I thought one weird trick to having good teeth was outright avoiding sugary drinks (soft drinks) and snacks. I always assumed avoiding causes was more important than remediation (teeth cleaning). Anecdotally, it seems reasonable.

That said, flossing gets rid of obvious stuck gunk, so I do that (with care to avoid damaging gums).

It's not just sugar. Soda water would do it too.

And it's not just carbonated beverages. Tea also is acidic

This is exactly right. If you really want to brush after eating acidic foods, at the very least swish generously with water first. Ideally, use a pre-brushing rinse that restores your mouth's pH balance.

To add to the other responses, using a straw can help, it gets the liquid past your teeth.

In part too the sugar with acid combo is particularly impactful, the sugar acts as an abrading agent. Hence, smoothies are something to take care with too.