Mars was far further along the path to being Terraforned before the Solar War, but with the vast majority of the Martian populace destroyed and all the terraforming equipment gone Mars has only recently had the opportunity to restart it's efforts.

So Venus has 500-600 years of effort due to pre war efforts, and Mars has maybe 100 given most efforts on Mars were rebuilding the underground cities.

So the reason is there and it was vital to the plot Venus be the only planet that escaped the war, since the war was supposed to be aimed outward from Earth, giving Venus a free pass.

(Also the atmosphere of Venus is 0.002% water vapour, the mass of the Venusian atmosphere is 4.8*10^20 kg, so the mass of water in that atmosphere is 9.6*10^17 Kilograms, and given 1:1 conversion that is 9.6*10^17 liters. Note Earth has a mere 1.35 *10^18 liters, Thus Venus has an amount of water not terribly disparate from Earths.)