because she is invisible, no one can prove that she does not exist (or indeed that she is not pink)
The Invisible Pink Unicorn is an illustration which attempts to demonstrate the absurdity of citing attributes and a lack of evidence as proof of a deity's existence.
Her two defining attributes, invisibility and color (pink), are inconsistent and contradictory; this is part of the satire.
The paradox of something being invisible yet having visible characteristics (e.g., color)
There are humorous mock debates amongst her "followers" concerning her other attributes, such as whether she is completely invisible, or invisible to most, but visible to those who have faith in her
Some arguments are quite elaborate and tortuous, satirizing the disputatiousness and intricacy of the theological debates that occur in many religions