@Vesper Shields are moderately expensive as far as medieval military equipment goes, mostly because they tend to take so much damage they don't last more than a couple battles and need to be completely replaced. They are simple to make, but when you're burning through that much material that quickly it can be very hard on the supply lines to keep everyone shielded.
Given that Shaon's people (the attackers) are still dealing with all the problems inherent in a civil war, it's feasible that they just don't have the resources (and maybe the money) to outfit everyone properly for a siege. Especially since the previous chapters established that they have something of a time limit to get this done.
Course, that's no comfort to the sorry bastards getting shot to shit.