Trade increased in Europe because of the interest in Eastern goods created by:

Marco Polo's book.

recapture of Constantinople by the Byzantines.

the Crusades.

both Marco Polo's book and the Crusades.

The Crusades is the answer. The Crusades, while bloody and dangerous battles, exposed Europeans to more goods than they had previously known about, for example new foods. They also saw that the members of the Islamic empire were trading further East than them, for example with China.