Venus had extensive volcanic activity in its early history (there is also some evidence of present volcanic activity). This volcanic activity released sulfur into the atmosphere and the high surface temperatures prevented it from being trapped on the surface as it was on Earth. The sulfur hence accumulates in the atmosphere as sulfur oxides which yield sulfuric acid in the presence of carbon dioxide, water vapor, and sunlight.
About the Author
Abhinav Jindal (he/him/his) is a graduate student in astronomy at Cornell University working on understanding the surface evolution of solar system bodies.