Removing routes from ISP DNS servers nationwide is often the first step in blocking content. It's easily bypassed if you know what you're doing, but most people don't and are stuck. China did this circa 2004, and AFAIK a lot of blocks in Vietnam are still done like this. I doubt Turkey has the infrastructure that more advanced censorship regimes like China and Iran use, so it's something of a poor man's solution (especially in a world of apps on mobile where the end user has no control over how data is fetched over the network).

Google DNS being just two IP addresses makes it a lot easier to shut down without collateral damage.