It seems like Eastern and Western europe share more similarities than differences, and there seemed to be parity in their technology and culture for most of history. Why is western europe, now, so much more prosperous?
Because, up until 1991 most of Eastern Europe was part of the "Eastern bloc" which was composed of the former communist states of central and eastern Europe. A combination of the intense poverty and injustices during the soviet era and the absolute anarchy that followed the fall of the soviet union crippled these countries, and most of them have yet to recover.