Science hasn’t totally answered this question yet. There are a few theories, but htey get revised quite often as we learn more.

But, you can get some idea by asking the question in reverse…why don’t other plants grow to be so tall.

Simply, it takes a lot of energy and is very hard for plants to get water to the top of a tree that tall. Redwoods must have some kind reason for doing this, so that it gives them an adaptive advantage. One of those advantages is that other plants don’t do it, so it reduces their competition. Also, they must have adaptations that allow them to do it more efficiently. The newest evidence suggests they can absorb water (fog) from structures near the tops of the tree, so they don’t have to pump all that water from the roots.

Also, they live a really long time, so they can grow longer. One reason they live so long is that they are fire resistant and have some pest resistant qualities, too. When a fire rips through a redwood forest it burns everything growing at the base of the redwood, and this in turn provides nutrients to the tree.