And that's why I think they copied the Newspaper Supplement format. And you are righ, it would have to be Weekly like a Newspaper Supplement to work effectively.
But we don't know what Publishing problems and issues they had to contend with either.
Well, at this point we were less than a year removed from the discovery that it was possible to create comic books, put them on the newsstand and actually have people pay money for them. New Fun was an attempt to see if there was a market for comics that were created exclusively for that comic book. It makes sense the format was the same as newspaper Sundays, at the time comics WERE daily 3-4 panel strips and weekly full pages, so they were just going with the established standard. Experimentation with the format and the realization that when you're creating original comics for a comic book you're not bound to the strips/sunday format like you are in newspapers was still a bit into the future.
One single page a month was the main issue - this was NOT what you got in the papers and NOT what you got in other comic books - and I'm pretty sure this was just a thing because they assumed they'd make the magazine weekly pretty soon. The editorial in both issue 1 and 2 is full of promises about a weekly schedule if sales are good enough.