Why did the English dub of Naruto stop?
Naruto aired on Cartoon Network and Naruto Shippuden aired on Disney XD. Apparently, Disney XD has dropped Shippuden from it's lineup but the Anime continues and episodes can be viewed on CrunchyRoll or Viz Anime, though the episodes are subtitled not dubbed.

Looks like, for those wanting Dubbed Naruto Shippuden, getting the DVDs/Blu-Rays is currently the only option. 

Update: Stream the latest English dubbed versions (legally) from Amazon Instant Video.