How did Mars become such an inhospitable desert ? Why ?

We know magnetic fields are related. This article will give you a lot more insights on what truly happened. Truer and eve more interesting than some strange worlds I have seen in sci-fi settings.

Credits: NASA/Goddard/MAVEN/CU Boulder/SVS/Cindy Starr

https://www.nasa.gov/press-release/goddard/2020/mars-electric-currents