What's the reason behind cabal (dependency) hell?

Question

How does dependency hell happen in Cabal-install?

I read the following at Cabal/Survival - HaskellWiki:

1. What is the difficulty caused by C

Answer 1

The problem is that you cannot link your program against those different versions of package C. A and B have to find a common version of C to use the same implementation of a certain function. One solution to this problem is OSGi but requires stuff like Classloaders which can be used to load different versions of package C in the same process w/o conflicts.