At least part of the answer lies in the nature of the downturn and in the policy responses to it. It is frequently claimed that the downturn was the worst since the 1930s. In the financial sectors of some major countries that seems clear, but for economic activity it is actually less clear than you might think.
For several important countries individually, the increase in unemployment and loss of real output was certainly equal to those in the most severe postWar recessions, but not significantly greater. Some respected scholars of US business cycles have suggested recently that it may be a bit soon to judge whether the recent period qualifies for the term “great recession”, at least in the US case.