Fuzzy-Wuzzy |
It's because tower shields are made of wood (page 277), and armor/shields of leather can be replaced with dragonhide, those of metal can be replaced by metals or dragonhide, and those of wood can be replaced with darkwood (page 555).
However, I don't think allowing wood => spiffy metals is likely to break anything. You'll have to do some work on the stats, of course. And an all-metal tower shield is going to have even higher Bulk than a regular one (which is why they're usually wood, I assume).