Because adultery is a very bad thing and he knows he can't face the people cos only the diehard ah bengs will support him.
Adultery is a bad thing but it's not a crime. He should have given a press conference with his wife beside him, admitted his indiscretions, begged forgiveness, and promised to work hard for the people of Hougang. He may not have been able to run in GE2016, but at least he could have stayed on for another 4 years and salvaged some of his reputation. I just don't get it -- was it pride, embarassment, or arrogance that caused him to act that way? Did he think he could just ride it out and wait for it all to die down? And why didn't Angela Oon try to clear her name if it was not true? Their behaviors really puzzling to me.