What would make a restaurant offer all-day breakfast? Some restaurants particularly fast food ones offer breakfast food only during, well, breakfast hours. Other restaurants however offer breakfast food throughout their opening hours. Some restaurants are specifically breakfast only so of course they do.

What's up with that? What would make a restaurant offer some food only sometime while others (besides breakfast restaurants) offer said food throughout opening hours?

I was thinking that this might have something to do with low demand or something, but then why do all-day breakfast restaurants, like, not go out of business?
This question makes a hidden assumption:  that all diners are on the same circadian rhythm.  A significant portion of the US population works nights, spans time zones, etc. For example, truckers.

That's one answer, that addresses the market.  There's also the question of inventory logistics for the restaurant.  Most eateries have to do extensive prep for a meal shift, and that includes thawing proteins, advance-cooking certain items, chopping, etc.  

Imagining an ideal world for the restaurant, they would prep exactly as much food, in various menu-item-proportion, as was ordered in a meal shift, and end up with none left over.  Meal prep left over would represent wasted inputs, and very often it is not able (or allowed) to be kept for the next meal shift.

Menu prep is "expensive" whether you end up selling one meal or a dozen.

Also, often "lunch cooks" have a different set of training/skill sets than "dinner cooks" than "breakfast cooks."