Conversation so far:
How did Kumar Sangakkara become captain?
In February 2009, Mahela Jayawardene announced that he would step down from captaincy. Kumar Sangakkara succeeded Jayawardene as Sri Lanka's captain.
What made Mahela Jayawardene want to step down from captaincy of the Sri Lanka cricket team?
Mahela Jayawardene announced that he would step down from captaincy in the best interests of the Sri Lankan team.
Did Kumar Sangakkara win the 2011 Cricket World Cup?
A month in advance of the 2011 World Cup in March, Kumar Sangakkara decided that he would resign the captaincy.

Question: Why did he step down?
Self-contained rewrite: Why did Kumar Sangakkara step down from captaincy of the Sri Lanka cricket team?