This is just tradition, personally I don't believe Russian writers of 18/19 centuries were better or worse than their counterparts living in Eastern Europe, for example. It is indisputable Russia produced some great writers and poets, such as Anna Akhmatova (but especially for poetry 70% is lost in translation). The reason that Tolstoi and Dostoyevski got famous is largely geopolitical; what we are doing now is just a part of tradition. I remember I relatively enjoyed reading Crime and Punishment, but when I started to read Brothers Karamazov, I started to have a feeling there are so many better ways to spend my life than reading these two bricks and this feeling only increased with time. YMMV.
Second, I am asking why should we treat Russian art as more important then everyone elses - or even as representants for Eastern Europe in general.