Same reason someone would vote for Barack Obama!
It's a novelty. Novelty (derived from Latin word novus for "new") is the quality of being new, or following from that, of being striking, original or unusual. Although it may be said to have an objective dimension (e.g. a new style of art coming into being, such as abstract art or impressionism) it generally exists in the subjective perceptions of individuals.
When the novelty wears off you are left with nothing but the reality that what you have is not what it was sold as.
Like Caherine says, the concept and the writeup that went with Obama, I mean the pet rock were pretty well written and cleverly done.