Because surely Putin would realise that coming out to defend him, and by saying he has a transcript of the meeting, could create more harm than assistance for Trump. And it is at least possible that the Russians have decided that Trump is just too unreliable for them - sure he looked easily manipulated before the election, but now they see the turmoil that surrounds him, maybe they want someone more predictable in the seat?
Or is Putin not as savvy to American political dynamics as we assume? It's not as if he has to worry about handling hostile press in his country.
Update: I see Andrew Bolt, one of the Australian Right wing media culture warriors who seems to have an exceptionally low interest in commenting on the patent turmoil in, and incompetence of, the Trump Whitehouse, briefly notes today that the Putin intervention will not help. He then ends with this bit of blind stupidity:
There is an unmistakable whiff of McCarthyism - mixed, of course, with an entirely insincere and self-interested outrage - to the anti-Russian hysteria now.Yes, of course, Andrew. If a Democrat President was refusing to disclose tax returns and financial information relevant to his or her ties to Russia, and had campaigners making suspicious trips to the country and later Russian sourced leaks were used to campaign advantage, you'd just shrug your shoulders about it and say "stop being hysterical".
Update 2: Homer in comments refers me to a post about Putin at Econospeak. It is interesting.