Because without an indisputable evidence it would only play for "RUSSIANS DID IT" narrative, "See! Because they say it was US means it was themselves!".
Russian propaganda machine keeps spouting lies about everything without much regard to credibility (firehose of lies), it's actually difficult to orient yourself in all the contradictory propaganda narratives.
It's just strange they keep largely silent on this, even though it seems like a golden opportunity. Why are they not attacking German society with "Americans are trying to freeze you, they blew up the pipes"?
Russia has an extensive network of influence campaigns within "unfriendly" countries, sponsoring parties like AfD, operating foreign language troll farms on Facebook and other networks, websites like Sputnik and "alternative media" etc. The list goes on. They absolutely do operate in Germany and try to split the society.
