Why? Not sure, unless you mean from a technical perspective.
Outpost 2 was somewhat designed with that in mind. There is an artificial damage modifier for Lasers and Microwaves when they're attacking walls. Plus, walls are not units, just a map cell, which means there essentially no memory to store anything specific to that wall. Hence they don't actually have any hitpoints. Instead, the max hitpoints assigned to the walls (in the sheets) is used along with the damage from the weapon to determine a probability of a wall being destroyed. It's done in such a way that normal walls must go through 3 stages, and can only go from one to the next when damage from a fresh weapons fire is applied. Hence, a nova, being only a single weapons fire when it goes off, can only take the walls through one such stage. (Lava walls and Blight walls would just be destroyed right away). I suspect the nova thing may have been more of an oversight of their memory limited existence.
And yeah, better defensive walls isn't actually that bad of an idea. One layer is too easy to shoot through, plus Outpost 2 makes it too hard to build multilayer walls. It also doesn't let you keep them repaired.