Each ship is given by a single organization.
The same organization has to dole out a ship to two classes for empire and republic.
That organization wouldn't buy 500,000 of one ship, and 500,000 of another, no it would save bulk and purchase 1,000,000 of a single ship.
It's for lore purposes, logical purposes, time saving purposes, and also I am sure like the above poster said to encourage other classes.