In comparison to what other group of leaders? Stalin lasted nearly 30 years as leader of the USSR, Krushchev over a decade, Brezhnev nearly two.

After Brezhnev they started to die a bit faster, but look at how old they all were. They were all in their 70s and 80s, an age when life expectancy tends to drop off quickly. Andropov died of total kidney failure, and Chernenko was already deeply (likely terminally) ill when he took over. He eventually died of emphysema and other complications of heavy lifelong smoking. After that was Gorbachev. It has been a long time since my undergraduate course on Soviet history, but I believe historians attribute at least some of the decline in the USSR to these geriatric rulers.

Here is a review of a book that explores some of these issues: "In addition, the failing health of Brezhnev and other top Politburo members became a barrier to change. With age and sickness, the Soviet leadership gradually lost the sense of reality that had tempered its conservatism in the past. According to his security aide, Vladimir Medvedev, Leonid Brezhnev was in his later years regularly under the influence of sleeping pills, which he took in excess for chronic insomnia. More and more frequently, the Soviet leader was literally unable to comprehend events around him."

After typing all that up, I am realizing you are perhaps referring to the Stalinist purges. Was that what you meant, or did you mean only the lifespans of the guys at the absolute top?

Like /u/pipkin42 said, after Brezhnev, they were all very sick and old. They kept choosing old, sick men to be in charge because they didn't want change. Gorbachev was the first Soviet leader to not have originated from the old guard of the revolution, thus his bringing about political and economic change to the USSR.

So in short, Soviet leaders died so frequently (towards the end) because they kept choosing old, sick members of a previous era.