I believe it breaks down to several factors: Choices The best discard cards create interesting choices
Antiquities was set around an Artifact concept even though all five colors were also used
We collect and pay this duty fee to our shipping partner (Royal Mail) for the following EU counties: Austria, Belgium, Cyprus, Denmark, Estonia, Finland, France, Germany, Ireland, Italy, Luxembourg, Malta, Netherlands, Portugal, Slovakia, Spain, Sweden
There are 6 rares and 1 mythic rare in the borderless frame break treatment (0.8% in total), 2 rares and 2 mythic rares in the borderless profile treatment (0.3% in total), 2 rares and 1 mythic rare in a retro frame (0.2% in total), and 1 additional borderless mythic rare (less than 0.1%)
You'll still discard a card as an additional cost to cast it