This is most likely because 4Kids Entertainment no longer have the rights to the Sonic X. Currently the series is still on Hulu, and Anime Dossier will thus undergo the process of updating the existing Sonic X videos to Hulu-derived, instead of YouTube-derived.
This is unfortunate news, but the series still exists online, just on a different medium with Hulu.
As Shakespeare put it:
Many strokes, though with a little axe, Hew down and fell the hardest-timber'd oak.It's move will be met with fresh videos straight from Hulu of the series to replace them. If you want to get a head-start on the series, you can check out the Hulu Sonic X page.
Sonic X via Hulu
