Probably they were worried that something about their definition might not apply to other star systems, and decided to kick that vote down the road a way once there's more data. As James noticed, the decision seems to deal mostly with the ambiguities at the small end of the planet definition, since the planet/star distinction is quite clear in our own solar system; but it is not at all clear when one considers the 200+ extrasolar planets known. A definition that applies to extrasolar planets would have to deal with the separate question of what constitutes a star or brown dwarf, and whether any of these are planets. And that's probably a whole other round of infighting.