The east has never been economically very strong, it used to be huge feudal-style farms and a bit of high-tech industry (planes, optics, trains, electronics in the 1930s). Not too different from now. Then there was the GDR which made things much worse.
It's not really West Germany that caused the weak economy of the East Germany. The handling of reunification did suck, but you can't just will a strong industrial base into existence neither.