There is a second, more urgent reason. Wood is porous. If the blood is spilled on it, recovery becomes an issue. I know in highly polished or sealed wood that becomes less of an issue, and I also know that marble and the like are also somewhat porous, but, imagine an unsealed piece of wood soaking up the precious blood like a sponge. No way to recover it and it could be open to descecration....
Thank you for that additional information. It only makes good sense.
I recall that period post VCII. Many churches, in their haste to erect an altar "facing the congregation", opted for wood which could be quickly assembled. Now, 40+ years later, the Vatican has issued a reminder to replace those altars with more permanent ones.