What is the main reason that inhaled vaccines are not in use today for flu for example? Is there some new technological breakthrough for delivery that was needed?
They are! Inhaled flu vaccines are generally live attenuated virus, which isn't recommended for everyone but is perfectly effective for a large segment of the population:
From a lay person's point of view, I am trying to imagine if there would be significant extra mass required to distribute a billion inhalable vs injectable vaccines. It seems like there would be additional mass and volume. Could that be one reason?
https://www.cdc.gov/flu/prevent/nasalspray.htm