Why are redwood trees that are grown in plantations not very decay-resistant?

The short answer is that growth rates are not everything with Redwood. In the growth rings, Redwood allocates tannins that help it resist decay. The closer the rings are together, the higher the concentration of tannins. Second-growth and plantation grown Redwood typically have large growth rings if managed under even-aged conditions, unless they are grown on longer rotations. Larger growth rings have less tannin per area than smaller growth rings. So if Redwood is grown to longer rotations under even-aged management or grown in uneven-aged management regimes, smaller growth rings lead to higher concentrations of the tannins that help it to resist decay.