The science is still ongoing but the best way to understand it is to ask the question in reverse. Why don’t other plants grow so tall?

-They die before they grow that tall.
-They fall over before they grow that tall.
-They die of disease or fire before they grow that tall.
– They are sensitive to annual variations in temperature or precipitation.
-They can’t pump water from the roots that high.
-It’s not a competitive advantage to spend that much energy growing that tall and/or other plants budget their energy for other purposes like reproduction.

-Redwoods live a very long time, so they can grow that tall.
-Redwoods have an amazing support system and the balanced flexibility help them stand up.
-Redwoods have amazing pest, disease, and fire resistance mechanisms including chemical defenses and thick bark.
-Redwoods are rather adaptable to variations, and may have systems that allow them to shut down rather than die.
-Scientists are still working on this, but it appears that redwoods absorb water from the fog, perhaps through rootlike structures in the tops.
-Redwoods have adapted to grow tall, possibly because other plants don’t. This can give them a competitive advantage in foggy forests to access sunlight. Because they live so long they don’t need to reproduce as much in a single year to be biologically successful.

The wood of the red wood does not rot. So this allows the tree to get old and massive.