Because you can only infuse two items from the same year. The frontier shell is from year 1 (despite having infuse) and the other shells with a higher defense are all from year 2. Its got to be a glitch or something, I mean why give it infuse if there is noting to infuse with it (the only other y1 shell also has a 3 defense). Maybe we need to get a new one from the terminal? Maybe the will patch it (by removing infuse ore making the frontier shell a y2 shell) in a future update