The reason for this is obvious: (Score 2, Insightful)

Politics. Pandering to the idiot vote. For Astrology believers, the 'downgrading' of Pluto was a slap in the face, provoking those feelings of religious outrage which politicians love to exploit. Millions and millions of voters in New Mexico have some sort of belief in Astrology, ranging from slight interest to passionate conviction. Many of those votes have just been guaranteed to those legislators responsible for this bill.

Being enlightened slashdotters, most of us have little appreciation for how stupid people really are. I am here to say that yes, they are that stupid.