Why why why bt make it such hard work to reclaim your voucher which was so clearly advertised.
Simple. The people that offer these sorts of rewards rely on a proportion of people failing to claim them. It's factored in when calculating how generous the offer can be. Too generous, and you end up making a loss (as Hoover discovered a few years ago).
As a result, you always have to apply for the reward, there's always a specific time window in which to apply, and you always have to keep something which people are liable to lose or throw away (in this case, your original order number).