Today someone asked the same question on X. Musk's response:
The strong gravity of Earth makes the physics of a fully reusable rocket with positive payload margin extremely difficult to solve, which is why it has never been done before.
Removing the mass of landing legs from the booster and ship by making the tower do the work of final velocity attenuation greatly improves payload margin.
This architecture also simultaneously substantially increases launch cadence, because the same arms that lift the booster and ship onto the launch stand also catch them, allowing immediate placement of the booster back on the launch stand and the ship back on top of the booster.
https://x.com/elonmusk/status/1845829120959156376
Achieving materially positive payload margin to a useful orbit with a fully & rapidly reusable rocket has eluded prior attempts. Many have tried to embark upon this path only to give up when it became clear that their design would have negative or negligible payload margin.
This is an extremely difficult problem to solve, given the strong gravity of Earth, whereas it is easy on Mars and trivial on the Moon. In the early years of SpaceX, I was not sure that success was even in the set of possible outcomes!
Fortunately, it just barely is, but requires doing unusual things like shifting the mass needed for final velocity attenuation and post-landing stabilization of the rocket (so it doesn’t tip over in wind) to the ground, rather than carrying heavy landing gear on both stages.
Truly amazing