There are many factors that seem to be responsible including the mere fact that the planets formed in different parts of the solar system from the sun. This caused the abundances of the compounds out of which they formed to be distinctly different. Venus received a lower abundance of water-rich componds, and a higher fraction of sulfer-rich compounds than the earth. Liquid water may have resided on the surface of venus for a while, but its ability to scrub CO2 out of the atmosphere was so limited that it could not overcome the rate at which volcanic activity was creating CO2, so CO2 built up and quickly you ended up with a greenhouse. The liquid water vaporized as surface temperatures rose and more CO2 was leached out of the surface rocks until we have the present atmosphere and temperature. Earth's chemistry was different, and it was farther out from the sun so that even larger bodies of liquid water could be present, and that made all the difference in the world. The early earth may have been quite a cold place, perhaps not more than a few degrees above the freezing point of water. As volcanic activity continued, the loading of CO2 in the atmosphere increased so that a greenhouse state was formed and the earth's surface warmed, and large bodies of liquid water appeared. The water then 'buffered' the CO2 and removed all but a bare trace of the greenhouse effect to the level we now enjoy!
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.