As far as I know the statue of limitations has not been reached yet. The reason he investigation was dropped was that Swedish law requires coercive measures to be proportionate, but I have no idea why they decided that it was too much in 2017 specifically. They had been talking about dropping the investigation since 2013.

Basically the Swedish authorities thought that going after him more than they already had would be too much for the level of suspicion and the crime.

Only the less serious charges had passed the statute of limitations.