Why did he lie?
Because he could?
Because he has and gotten away with it and found it profitable?
He had to have examples of what lies can do for you and no fear of punishment or consequences to be this bold and lie before a nation. He is the son of the POTUS.
Then again, the lies Trump sr has told are pretty outstanding  too.

I'm sure we don't know the half of it till we see Trump sr tax returns. 
The Trump's have been dealing with Russian government and the Russian billionaires for years.