Says he found "a more exciting job". According to Russia Today:

Yury Lonchakov was scheduled to go to space in the 2015 International Space Station (ISS) mission on the Soyuz TMA-16M ship, alongside NASA astronaut Kjell Lindgren, Japanese Exploration Aerospace Agency (JAXA) astronaut Kimiya Yui, and Russian Federal Space Agency cosmonaut Oleg Kononenko.

“He came up to me and said that he found a more exciting job than working in outer space. And wrote his resignation letter,” chief of the Cosmonaut Training Center, Sergey Krikalev, told Interfax news agency on Thursday.

“Frankly, we counted on his participation because he was not just in the unit, he was assigned to the crew," said Krikalev, adding that he does not understand the decision.

Lonchakov’s retirement sent the Russian blogosphere into a frenzy, with many questioning which profession could possibly be more interesting than being an cosmonaut.

Any speculation what could be more exciting? Maybe a private venture into space? Something on Earth?