Why have the Americans developed space shuttle according to the passage?

Ever since the Apollo moon landings, the idea of human expeditions to the Mars has come up repeatedly as the kind of grand adventures that would give focus, purpose, and inspirational lift to the space program. But confronted with the cost — estimated at $ 150 billion last week — presidents starting with Richard Nixon and others have flinched and resorted to halfway measures like the shuttle.

It’s a trick that time and memory play on us: Comedy — like glamour or courage, is never what it used to be. The present may sometimes make us laugh the hardest, but we like to tell ourselves that the past makes us laugh the best. Today’s emphasis on physical comedy is perhaps a natural reaction to the more cerebral comedy of the past two decades.