Simply because I never thought of an alternative to 'natural selection'. One should, infact, be aware of the fact that nature should be treated as an upredictable and contingent thing.
It is only then that one can understand 'natural selection'.
As far as the word 'evolution' is concerned, transmutation (of species) is a better word. This is because 'mutation' in its biological sense is a strictly contingent/random process.
This 'contingency' coupled with the 'selection' of 'nature' is the very essence of EVOLUTION.
If anybody wants to have a crisp definition of the theory of evolution then to some extent, this title will serve the purpose well:
"Transmutation of species by Natural selection".

Not to forget that 'species' is an ill-defined term. The ordinary biological definition is a just a working statement. Interestingly, this illness inherent in the term 'species' is due to the very process of random, naturally selected transmutation itself.

Yasir.