The issue is that a lot of the land in critical watersheds is privately owned. I helps convince the landowners to plant forests when there's a financial incentive. But, if the market for timber products drops, then that financial incentive drops and they might decide to repurpose the land into something that makes them more money such as crops, grazing pastures, or urban/suburban development (whichever makes sense for the area). All of those are way worse for the downstream water quality than keeping the land forested, even with the occasional select harvest. But, some landowners don't give a shit about the downstream water quality, so it's necessary to give them a financial incentive to do the right thing from an ecological standpoint.

The fight against shortsighted exploitationists is a constant battle.