The answer to that question is found in the 4th book of Moses, The book of Numbers, chapter 14 verse 34, in 'Parasha Shlach.' Please read the complete chapter that details the account of the Israelites spying out the land of Canaan.
Verse 34, ‘[The punishment] shall parallel the number of days you spent exploring the land. There were forty days, and there shall be one year for each day, a total of forty years until your sin is forgiven. You will then know how I act.'
This is simply because of their disbelief in the remarkable power of G-d, considering all that G-d had done for them to that point. The period of forty years was for the unbelieving generation to die off, so that a new generation could rise up and enter the Holy Land of Israel.
