Always poorer is a very big claim. Some of the earliest cities and worked metals in Europe lie in Eastern Europe, revealing the considerable richness of people at that time. Ancient Greece arguably wasn't the poorest in the continent either. Byzantium outlived the Western half of the Roman Empire by close to a thousand years. The Ottoman Empire covered a sizeable part of Eastern Europe and was quite wealthy for a while. The First Bulgarian Empire was once one of only three empires on the continent. The Hanseatic League, one of the biggest trade alliances in history extended all the way to Veliki Novogorod - and it wasn't called "Veliki" for nothing. The Polish-Hungarian Kingdom and the Polish-Lithuanian Commonwealth were also major forces in the region. Then there is the Habsburg Empire...

More recently however most Eastern European countries have been in general poorer than Western ones ... due to a number of factors. I think that the influence of the soviet union and communist rule is by far the greatest factor, as being most recent. The fall of the soviet order also hit these countries pretty bad, and the transition to capitalism led to some economical trouble and gave the opportunity for a lot of shady and corrupt people to obtain power - something that to this day plagues the region.

Before communism came to the region, an important factor for at least the Balkans is that many of the countries there had gained their independance relatively recently from the Ottoman Empire. And have been ravaged by two Balkan Wars, two World Wars, a Serbo-Bulgarian war and probably a few more... And as part of the Ottoman Empire those countries were subject to the slow modernization and crises characteristic of the late Empire.

First of all you have to define "Eastern Europe". Majority of what Americans call "Eastern Europe" is more correctly called "Central" and Southern" or "Southeastern" Europe. Geographically eastern Europe is Russia/Ukraine. Poland, Czechoslovakia and Hungary are central Europe, the Balkans are southeastern. Remember also that before 1918 and 1945 they were under German/Austro-Hungarian rule. I will therefore keep using quotation marks to differentiate political and geographical terms.

Secondly it is not true that "Eastern Europe" was always poorer. During the times of Polish-Lithuanian Commonwealth in may ways you could argue for a comparable standard of living. The drop-off is first noticeable when industrial revolution starts. "Eastern Europe" was slower to industrialize because of its relative strength in extensive agriculture and lower population density. On the other hand contemporary Czech Republic was at the industrial heart of Austro-Hungary and was considered a rich region even by western standards. Southeastern Europe was under Ottoman rule and as a result it struggled for political reasons as well as due to constant wars for independence. The real drop-off can be seen during WW1 when parts of "Eastern Europe" such as modern Poland and Slovakia as well as parts of Balkans were devastated by prolonged and heavy fighting. In particular Poland was hit very strong because before WW1 those parts were among the most industrialized parts of Russian Empire (although not as well as western Europe) and as a result of shifting frontlines both Russia and Prussia destroyed and pillaged the region. The devastation there was comparable to what happened on the western front but more extensively because the frontlines actually moved. Then after WW2 which brought about similar destruction the region went under Soviet rule which crippled any economic development for a long time.

Quite interestingly today the pre-WW1 differences are still seen although in different proportions. Czech Republic is richer than western Poland, Hungary and Slovakia etc.