I had not realised that Gamescom is so important to be honest. I have visited both events in the past and because of the megatons that usually happen at E3 it feels like the more important event of the two. That said, there is no guarantee that CD Projekt won't skip Gamescom.
It might also be an issue of it not being so well known in the US, they still need to be warmed up to the Witcher games. But I do think Gamescom is detrimental to TW3 marketing, in the end Europe is where CDPR's core audience is.