The parthians were the beneficiary of the Romans wrecking the seleucid military and then repeatedly intervening in seleucid succession and preventing them from conquering Egypt. The seleucid empire was briefly in a period of instability before the romans, but they had recovered and were close to their peak well before Magnesia. The first Roman-Syrian war completely wrecked them.Seleucia was the first Seleucid capital.As for why the empire fell,it has a large part to do with the Parthians,and before that,the Bactrians.Large parts of the empire broke from Seleucid rule even before the Romans came knocking.
John Grainger is a good read on Seleucid history.