Because the time when they were either first formed, or the most active, was long ago which now translates into a great distance of billions of light years. The actual number of quasars is pretty small. It is believed that only a few 10,000 may have been formed in our observable universe since a billion years after the Big Bang. This is about equal to the number of very massive 'giant' galaxies, of which there are quite a few in our neighborhood out to several 100 million light years. They represent a rare phenomenon compared to the perhaps 80 to 100 BILLION other galaxies in our visible universe, and their Great Age ended several billion years ago.