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.

Two years ago, in the Qatari capital, trade ministers agreed to begin global trade negotiations driven by what is known as the Doha Development Agenda, which put the question of development at its core. It is widely acknowledged today that trade is a vitally important element in any program for development, as it can deliver benefits to developing countries worth many times more than all the development aid they receive.