Why doesnt mars have an atmosphere?

Answer 1

Mars does have an atmosphere, though it's quite different than ours on Earth. It's composed mostly of carbon dioxide with only negligible amounts of oxygen. Also, the pressure at surface level is roughly 100 times smaller than pressure on Earth's sea level.

Mars isn't capable of supporting much denser atmosphere, it's smaller and less massive than Earth.

There are no plans currently for doing terraforming on any kind, actually there are no serious plans for even a manned flight to Mars.

Terraforming would be a massive engineering project, and as for now, we don't posess technology nor resources to undertake it.

Answer 2

Mars has a very thin atmosphere. The reason it does not have more is quite simply gravity. The gravity on Mars is not sufficient to "hold" onto an atmosphere and it is "blown" into space buy the solar winds.

Answer 3

Mars does in fact have an atmosphere, but it's extremely thin and mostly consists of carbon dioxide.

The most likely explanation is down to the fact Mars lacks a magnetic field to protect it from the solar wind. Without this shielding the solar wind can "blow off" gases in the ephemeral upper layers of the atmosphere, over billions of years this has thinned Mars' atmosphere to almost nothing.