There were actually strong political reasoning behind that, it is just nobles marrying into an already existing political elite so they have legitimacy of government. The commoners will accept the new Greek overlords because they're married to the old overlords.
Plus political elites from one group have more in common than political elites from another group than either group have in common with their peasants.
Alexander the Great also forced his generals to marry Persian women. He had some “cultural fusion” plan going.
There were actually strong political reasoning behind that, it is just nobles marrying into an already existing political elite so they have legitimacy of government. The commoners will accept the new Greek overlords because they're married to the old overlords.
Plus political elites from one group have more in common than political elites from another group than either group have in common with their peasants.