Why was columbus' voyage described as a successful failure?

Success for Spain: Columbus' voyage opened up the "New World" to European exploitation, which would generate untold wealth for Spain and other nations through overseas colonies.

Failure for Columbus: Columbus was looking for a way to reach the East Indies in Asia by sailing westward across the ocean. Obviously he did not acheive that goal. However, the crazy old coot went to his deathbed insisting what he had found was Asia, refusing to accept the abundant evidence it was not.