You have to go all-out for the Opening Ceremonies because first impressions are remembered much more than anything that follows. It is far more important to make a good first impression than a second, third, or even last impression.
Plus there is the excitement of hope & expectations before the Games begin. There is a lot of energy & anticipation at the beginning for both the athletes and the audience. At the end, many athletes have already gone home, there is nothing exciting to look forward to for the fans, and it's nothing more than the forced fun of an over-extravagant party.
They did try to hold something back for the closing ceremony (if The Spice Girls is your idea of top entertainment). If some people liked it, fine. But I don't think I've ever seen a Olympic closing ceremony that was superior to its opening ceremony. So I wasn't surprised it didn't deliver.