What problem primarily drove NASA to seek collaboration after the Space Shuttle program ended?

Answer

Reliance on Russian Soyuz rockets for astronaut transport and high operational costs

Following the Space Shuttle's retirement, NASA became dependent on Soyuz rockets to transport astronauts to the ISS. A major goal of the subsequent programs was to resolve this dependency while reducing overall operational expenses.

What problem primarily drove NASA to seek collaboration after the Space Shuttle program ended?
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