Because landing upright works for Mars and the Moon; It also scales well to larger payloads.
Elon is being sneaky and developing a way to colonise Mars but getting paying customers to fund the necessary R&D.
The NASA method of using a heatshield, parachute and skycrane to land the Curiosity Rover on Mars was ingenious but at the maximum weight limit of what that method could achieve. Any heavier and it won't work.
A propulsive vertical landing on Mars could go much heavier. Much much heavier. Weights of 50 tons to the surface of Mars are being talked about.