Outisder's view: Beacause that what the respective contracts say, or would one prefer to work for a company that does not honour the signed?
Whether or not is it possible to have a motivated, amalgamated workforce with such a wide range of contract conditions for the same work or more work, performed to the same standard or even better standards, is a different question. To which the answer is a simple flying no.
Funny that the managers and owners will claim to be absolved from this mess, saying it is what the workers agreed to and wanted / voted for through the unions. Not so funny they're actually right to a large extent, most definitely there's no way of fixing it.
Have the same shirt, different country of origin.