I don't know, but maybe because with this definition the four vectors are positively oriented:
In[8]:= v2 = {0, 1, 0, 0}; v3 = {0, 0, 1, 0}; v4 = {0, 0, 0, 1}; cross = Cross[v2, v3, v4]; Det[{v2, v3, v4, cross}] Out[12]= 1
