Because it is part of today's daily specials. As a rule, no more than 1 or 2 of these can be ordered per person. As soon as the offer expires, you can easily order 4 pieces at once ;)

Had the same problem with 2 disks.... (Only have 2 Bays).
But I was able to tell my work colleague to buy one too... so I'll have two of them tomorrow:)