First, the ferritin protein is common to all human beings. Indeed, some type of ferritin is found in most living organisms. Ferritin is found all over the body. Thus, it would be a good carrier to get the Covid-19 antigen to any and all parts of the body. This might be helpful in combating some of the non-respiratory pathology from this virus.
Second, the complete ferritin protein is assembled from 24 subunits. In theory, each of these sub-units could carry a different genetic variant of the Corona virus spike. Also, 24 challenge spikes may elicit a greater and/or more rapid reaction from the body's immune system. 24 "invaders" may get more notice by the immune system than just one or two.