YouTube's community is much larger than Newgrounds'. This has its own pros and cons.
Large Community Pros: More, fresher viewers with a wider range of tastes and much more focused on the content of the upload.
Large Community Cons: More kids, more advertisements, more mainstream content, no porn.
Small Community Pros: More maturity, higher standards for technical quallity, fewer advertisements, porn.
Small Community Cons: Harsher critics, fewer viewers, freaky/weird content.
Youtube - Bright color scheme - Nice, immature audience.
DeviantArt - Bright color scheme - Nice, somewhat mature audience.
Newgrounds - Dark color scheme - Cynical, mature audience.
Coincidence, or social/psychological factor?