What enabled wheat and apples to become key crops in Washington?

Irrigation

Explanation:

Washington's region typically receive very little rainfall. If the farmers relied on nature alone, many parts of Washington will most likely have to experience constant crop failure due to drought.

Irrigation allow these farmers to collect water from Columbia, Yakima, and Snake River watersheds. The water that they took from irrigation made them able to provide enough water for the crops to grow without having to rely on rain. Because of this, various types of crops such as apples and wheat can grow pretty well in Washington.