Yeah Starfox 2 is the only game in the series that feels like a proper sequel to me. As for it being short, I doubt that was why it was dropped, it's not really any shorter than the first game, and it plays out differently each time so it's highly replayable.
That's true, the 1st game wasn't that much longer either. It would take about 1.2h to finish SF1 however SF2 can be completed in 44m and as ARandomGamer posted, SF2 was re-playable many times due to the different actions you may take. Plus in an era where Saves weren't standard, long games wouldn't be ideal to finish.
The music was also great, i don't think many games can top Star Fox 1 in the music department but Star Fox 2 did add more great themes to the series.