Why do Mangoes have a single large seed, and not multiple small ones?
It is an interesting question and it applies to other fruits with big seeds as the avocado as well. It is very likely that these fruits evolved to be spread by large animals (like big birds or large mamals) which existed for a relatively long time, but are extinct today. These animals swallowed whole fruits and excreted the seeds afterwards (most of these seeds are slighty laxative) together with dung to support the growth. 

They are called "evolutionary anachronisms" and there is a very interesting book about them, called "The ghosts of evolution: nonsensical fruit, missing partners, and other ecological anachronisms." (which can be found as a PDF online)
Two interesting articles about this phenomenon can be found here:


The Trees That Miss The Mammoths
Anachronistic Fruits and the Ghosts Who Haunt Them