Why did the Papal conclave use a Faraday cage in 2013 when selecting the new pope?

When you read up on the source quoted in the article, you will notice that it apparently isn't talking about a "real" Farraday cage around the Sistine Chapel (putting a whole building under a wiremesh dome would be ridiculous even for the Catholic church) but rather about a figurative one in form of:

the installation of equipment which blocks any electronic signals from getting out of the Sistine Chapel.

The author doesn't seem to be very tech-literate (see second to last paragraph), but it appears like he is talking about GSM jammers or similar devices which make cellphone communication impossible though active interference, not passive blocking. Such device are available commercially over the shelf for affordable prices. By the way: such devices violate broadcasting regulations in many countries making them illegal. But the Vatican is a sovereign state, so they don't have that problem.

The main motivation for this measure was that during the last conclave in 2005, the election of Pope Benedict XVI was leaked to the media before the official announcement. This is what the Catholic church wanted to prevent from happening again in 2013 and this is why they took technological measures to prevent anyone inside the conclave from communicating with the outside world until the official announcement.

Leaking information later was a secondary concern.

A Faraday cage will successfully block both incoming and outgoing radio communications.

So all you need to guess at is what someone might wish to do. There are a few possible, but to me the two most likely are:

• betting: getting information out early could make a lot of money
• threats: to some, the papal result is significant, and some parties may wish to put pressure on the cardinals

But in reality, it's just a way to prevent distractions or communications of any kind, thus ensuring the cardinals will be focused on their role.