I imagine the reason you don't see that many older Mercedes in Germany is the same reason you don't see that many 25 years old chevy's and fords in the states also, unless it is in a classic car category.
As for other eastern european countries, the population in those countries were driving russian and soviet era cars until the berlin wall came down, and any old benz from western europe ended up there, rust and all, and probably there aren't that much network of parts dealers there.
Also Mercedes is/was considered a high class car here, and owners tended to take care of them much more than domestically made cars.

1988 300SE
1985 Peugeot 505GL
1999 Jeep Grand Cherokee
1996 Mercury villager