Why Christians do not believe that Moses wrote the entire book of Deuteronomy?

Most Christians would hold that Moses wrote Deuteronomy, although the obvious difficulty about the account of Moses' death causes questions for some. What must be remembered is that in claiming substantial Mosaic authorship of the Pentateuch, it does not necessarily mean that Moses wrote every single Word. Nor does this affect the Word being God's inspired Word if someone such as Joshua added the epilogue about Moses' death.