Why are C4 plants mainly tropical plants?
This is not my field, so the following is based solely on a brief reading of Internet sources.

The Wikipedia entry on C4 carbon fixation states:


  When grown in the same environment, at 30°C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277.


However it does not explain why this is so.

In contrast, the section in Berg et al. focuses on the energy losses caused by photorespiration and makes the following initial point:


  …the oxygenase activity of rubisco increases more rapidly with temperature than does its carboxylase activity. How then do plants, such as sugar cane, that grow in hot climates prevent very high rates of wasteful photorespiration?


The answer to this (didactic) question is the C4 pathway (as described in full in that text), but the answer to the question posted here  is that “tropical plants employ the C4 pathway because of this increased wastage of energy through photorespiration at the higher temperatures in which they live.”