The power was off at my house. I shut off the main breaker. Shut off all 240 volt panel breakers. Plugged a generator into a 15 volt wall receptacle. All fixtures on this circuit functioned. I expected electricity to flow to the other open 115 volt circuits. It did not. Any reason why?

Because if you are in the US and Canada... you have TWO power legs that come in to the main panel... and they aren't connected to each other.

Thank you

Sounds like you made a "Suicide Cord", If so, If the Generator was running and cord pulled out, prongs would be Live. An Interlockkit.com it best/safest way. Keeps from backfeeding. Not sure your situation, If Puerto Rico, and in an area yet to be restored, and Main power is disconnected, you could attach a jumper across a Dryer? 2 pole breaker, making the other half of the receptacles work. Can you explain more? You may have a LARGE Lawsuit should someone die. Electricity can Kill.
Good Luck