Why is saying 'It's going to get worse before it gets better' a bad thing especially when it's the truth?

I think the problem is that he didn't admit to this before the election. He was asked repeatedly, is the economy so bad that you'll have to alter your campaign promises, and he always said no. Now, AFTER becoming President, he tells us the gloomy side. Why wasn't he honest BEFORE the election? This suggests maybe he won on false pretenses.