He made that promise as a last-ditch attempt to win votes. He said that IF ELECTED, he would be the people's voice in Cabinet, and he would reform the PAP from within.
However, how in the world can you reform the PAP when you are voted out and no longer part of it?
I think many people wanted him to run for President, as that would have given him a chance to fulfil his promise. I believe he will run for President one day. He was blocked because the PAP hand-picked Tony Tan, who is known to be one of LHL/LKY's closest confidantes and allies.