When it all comes down to it, your record card, housing availability, and location needs are the determining factors. At any given time, there are about 7,000 people on the CP or ICP. About 15,000 students go through the programs each year. Managing housing availability between the CP, ICP, and the many Operating Participants creates a lot of back and forth with available spots.
When talking about spotless record cards, they really need to be that, spotless. For Fall 2011, acceptance rate for the CP/ICP was high, putting a tight limit on who could extend. Those with 1 point (equivalent to calling in once, or being less than two hours late twice during their entire program) or more were automatically excluded as they could only take about 300 extensions.
Standards change each year, but the best way to make yourself competitive for extending is to have a spotless record card.