Whereas Shuckle needs to run HP to be effective.
Basically, walls have 3 stats where they want to invest in. HP, Def and SDef. You can only choose 2 or be a mixed wall, which is not that effective in the HO-meta we currently have, so you need to choose.
Shuckle is a bit awkward in this regard though, so 252 HP and 128 in both defences is something I can understand, but for other mons, their typing or stat layout lends them to defend on one side of the spectrum better than the other (another exception is Amoonguss, but idk in which tier that thing is).