There are a lot of small reasons.
Primarily, all trailers are going to be built on older builds by default because of the editing labour involved. In this specific case, Yu Suzuki apparently confirmed to IGN Japan's Esra Krabbe that the Gamescom trailer is an older build, and the demo is a recent one with some brushed up visual elements.
Why there's apparently such a stark difference has to do with data and detail loss in their video production pipeline, IMO. Depending on how they're capturing and later exporting the footage in regard to resolution, bitrate, and codecs; a huge amount of fine detail and visual data can be washed away and made worse by YouTube compression once it goes public. I also believe they're capturing from the PC version, even if there are PS4 symbols in some of the trailers.
Honestly even if they did do everything perfectly in their video production pipeline... it's not going to compete with seeing the game running in front of you at 4K, on a nice monitor.