Why are the crusades sometimes called "successful failures"?

I would say it was considered a success because new discoveries were made, pope's powers increased, and Christianity was spread farther; it was a failure because they lost the Crusades.  

The crusades were considered a failure because
the christians did not gain Jerusalem back so the muslims still owned the land.

The transcontinental railroad provided a means of fast transportation. it cut a 6 month trip into 6 days, created jobs for many people and encouraged immigration. the homestead act played a huge role in western expansion; it allowed settlers to begin a fresh start, and gave them land in exchange for their residence for up to five years and a small fee.