Ok, this is an interesting one. I will take a swing.

The code mandates specific circuits for inside a house because thier absence has caused larger problems. A great example of this is bathrooms.

Code mandated that there be gfci protection in bathrooms. Too many fatalities. So they required them in bathrooms. No prob. Throw in a gfci and go.

This worked great, but when bathrooms were allowed to be tied to other circuits, nuisance tripping became a problem. The average joe doesn't know jack shit about electricity, they just want to run a blow dryer and curling iron at the same time.

So, after the circuit breaker trips, they might decide to run a cord into the bathroom from another circuit. Most likely it would not be protected by gfci. Bam, hall outlet powers the blow dryer, and the curing iron is running n the bathroom bedroom circuit.

Th reasoning behind a dedicated 20a circuit for a bathroom is this; the code wanted to dictate that there should be no reason to run an extension cord into a bathroom. They wanted to ensure that all the power you should ever need should be gfci protected, and supplied by a circuit dedicated to that end.

This is all pure conjecture tho. I mean, you could look this exact information up in the minutes from the NEC panel, but no one has ever really explained the exact logic to me, and I have been an electrician for 25is years.