A Pope is elected when two thirds plus one Cardinals vote for person on one ballot during the conclave. There are usually three ballots each day, which means that Benedict was probably elected on the fourth ballot–which is relatively quick. This means that there seems to have been a general concensus among the Cardinals, but we never can tell what really happened. Maybe the most liberal Cardinal of the bunch was elected on the first ballot, and then declined–who knows.
Also, there were much stronger reasons than the Tridentine Mass to elect Benedict. First, the world faces a moral crisis, and the safest thing to do in such a case is to elect relativism’s most outspoken critic, rather than elect a more progressive Pope, as many were pulling for (believe it or not, some of the Cardinals of voting age are essentially pro-choice). Also, Ratzinger was certainly the most intellectual Cardinal, and the most experienced in administrative matters–he was capable. Third, he was an all-around Holy man. Fourth, it isn’t easy to fill the shoes of somebody like JP2, and the cardinals were probably looking for someone humble. On that same note, they probably wanted a transitional Pope–someone who could bridge the gap between JP2 and the next long-reigning Pope, and begin a trend toward tradition and order that JP2 paved the way for. Think about it–if Benedict does about 10 years as Pope, and the next pontiff continues his initiatives, it doesn’t look like one stubborn man trying to hold on to fading traditions, but but more like undeniable truth. There were a million and one reasons to elect this man, who to me was far more qualified than any other person in Rome that week.
Also, it ought to be said that Popes often “set up” for electing the kind of successor they would like by appointing like-minded Cardinals. Most of the Cardinals voting were appointed by JP2, and so the choice of Ratzinger is more a reflection of whom JP2 wanted as a sucessor than anything else. The previous Pope was a very smart man, and I’m pretty sure that he had a fairly good idea about who his sucessor would be.
Similarly, Benedict is probably “setting up” the College of Cardinals to elect a Pope that would meet his standards of approval. This may sound contradictory to the fact that the conclave is guided by the Holy Spirit, but isn’t the Pope guided by the Holy Spirit as well? Also, I recall reading somewhere that in former days, Popes could name their successors themselves while they were still alive.