Plus don’t militaries have radar anyways for air defense? How is it possible that no radar from multiple countries picked up a silent Boeing 777 flying to nowhere?
Primary radar requires relatively a lot of power, and has other issues (e.g. target correlation) that make it of limited use for ATC purposes during enroute flight. If such radars exist, for military purposes for example, ATC may not have access to them as it's not really essential for their work. There are also large coverage gaps over oceans and in remote areas where nobody cares to monitor. These radars would only have a range of a couple hundred nm maximum, so they would not see deep into the Indian Ocean either.
So called 'over the horizon' radars also exist, but require truly massive amounts of power, not to mention capital to build, and also have very low spatial resolution. Apparently Australia's $1.8bn system was aimed in the wrong direction at the time of MH370's disappearance, or it might have had some low precision data to contribute.
But it does break the bank. Radar may not be new tech, but it is still expensive to operate. Even the US is removing radars. To be fair, the US has thousands of airports (and even more small airfields), but it has a huge area to cover.
Also, the military angle is covered in the article.