Why has Venus's atmosphere not been stripped away by solar wind? This answer on Space Exploration to a question about Mars says that one reason Mars has such a thin atmosphere is because it lacks a magnetic field to protect it from the effect of double solar winds.
Here MBR explains that Venus does not have a magnetic field.

Image credit: ESA
If this is the case, then why has Venus's atmosphere not been stripped away by solar wind like that of Mars?
Venus has a strong ionosphere that protects it against violent solar winds. So, even though Venus has no intrisic magnetic field, it has an effective, induced magnetic field due to the interaction between the solar winds and the atmosphere, that protects it against solar winds.

Venus atmosphere is thick enough to have a consequent ionosphere, that would be the difference between Mars and Venus (and Venus was able to keep a thicker atmosphere due to its greater mass, contrary to Mars).

Sources:


Luhmann &amp; Russell (1997),  on Venus
Luhmann &amp; Russell (1997),  on Mars