Presumably because the contracting parties want the contract to be interpreted and enforced by the courts of a country in which they both do business.

It's not unusual, I think, for big international companies that already have assets and legal advisers in the UK to specify English law and English courts for a contract, but if a small Greek and a small Italian company were to have a contract in English, it would, I imagine, just make a legal dispute much more dififcult and expensive to resolve (whether or not the contract specifies English law and English courts: different problems in each case).