1. Most of the changes in the east were applied in 1939, when Byelorussian SSR and Ukrainian SSR were enlarged with land taken from Poland. The line, that was agreed by Molotov with Ribbentrop and adjusted later that year, closely resembled what was to become Polish eastern Border (with exception to Białystok and Podlasie - these were returned in 1945).
2. Moving Poland westwards meant that it will create tension between Poland and Germany (Up to this very moment (26.08.2018) both countries have signed many bilateral agreements, but no Peace Agreement after 2 WW). This meant that Polish People's Republic needed Soviet Protection. East Germany alone had 19 millions people compared to Polish 23,6 millions^([1946]). Additionally West Germany had less than 50 millions people.

Normalization between PRL and West Germany only happened after 1970, when the Germans agreed on Odra-Nysa Łużycka border, but only temporarily. The case was to be revised after the unification of Germany.

Edit: Extended some parts.