I don't know why he has not imposed the sanctions. On the other hand, Russia is an important country. Neither the situation in Ukraine, nor in the Middle East, nor perhaps in North Korea, can be improved without Russian cooperation. Maybe there is some sense to not allowing the relationship between the two countries to become further estranged.
Russia has been meddling in other countries domestic politics big time for a long time, at least since the 19th century. No sanctions will end that. It is our job to prevent the meddling from having much of an effect. Rosenstein reported that it had not. Obama claimed that Russian meddling was not and could not be a factor in our elections. Based on what we now know, claims to the contrary are not convincing. In the end, it is our job to prevent Russian meddling from being effective.
The US is in a very poor position to object to other countries meddling in the US because we have a long record of meddling in the domestic politics of others. Furthermore, other countries constantly meddle in our politics through lobbying and foreign campaign money. Other people hack us and we also hack them. Everyone needs to defend themselves and complaining will do little good. The anti-Trumpian narrowly partisan approach to the problem of foreign meddling and hacking of the US is counterproductive.