Short answer: Because the "national governing body" for the Catholic church, known as the papacy, is making management changes so as to reboot and allow the "cement to dry" in regards to the sexual abuse reforms that the church has implemented.
In modern times Catholic Popes' have chosen to remain in office till death do they part so as to resonate their pro-life position and demonstrate that it is God's will as to when they are "retired." It has been that way for about 600-years.
Pope Benedict XVI was deemed Pope for the Catholic Church in 2005 after the passing of Pope John Paul the II. Soon thereafter Pope Benedict was engulfed in a sea of sex abuse scandals which he reacted by enacting reforms and making easier to defrock priests 
However, parishioners in several cities such as Los Angeles are still enraged because new details have come to light. Details that illustrate cover-ups and misdirection by Cardinals and their underlings which have caused untold hurt and suffering. Though the Catholic church instituted reforms and made strict child abuse prevention policies it is hard for the parishioners to trust that these policies can or would be implemented by the same people who let it grow out of control in the first place. With Pope Benedict's resignation the church reboots and the sex scandals become the last Pope's problems. Let's hope he takes some Cardinals with him.
From USA Today:
VATICAN CITY -- Pope Benedict XVI announced Monday that he will retire from his 8-year papal reign, citing age and declining health for becoming the first pope in 600 years to resign the post. [...]
The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.
This resignation will allow new reforms to "cement in place." It will allow a new Papacy team some breathing room so as to take decisive actions to get the church back on track in the eyes of the parishioner. The USA swimming board should take note and put their scandals behind them by doing likewise.
 Wall Street Journal February 11, 2013, 10:49 a.m. ET