system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] How did Pokemon that weren't Fairy type before suddenly gain fairy's resistances and weaknesses?
gpt
Classical evolution. 

There were variants with the dominant fairy type already, but they were a minority in the population.  Occasionally one would get caught and oddities would be seen.  However, it was anyways written off as a quirk before fairy was recognized.

Meanwhile the classic examples of the Pokemon species you were likely over caught.  This lead to an increased proportion of the gene pool being faeries.  So successive generations were quickly overwhelmed by fairies.

The speed of which seems to imply it is a dominant trait, but environmental pressures weighed heavily against it.