Devoid of emotions, artificial soldiers could follow commands without question and be sacrificed in great numbers without ethical and moral implications.
And then after their mission was over, they were to die off relatively quickly given their accelerated aging. Droids, on the other hand, displayed no such issues. Millions of living, sentient beings were intentionally created for one purpose: to kill. The mere idea of a genetically engineered a clone army went against everything the Republic and the Jedi stood for. Although clone troopers were some of the best warriors in the galaxy, a droid army would've been easier to produce and command, without a myriad of financial, logistical, and moral issues that accompanied the controversial process of cloning.
In the Star Wars prequels, the Grand Army of the Republic was made up entirely of clones, instead of battle droids, and the Republic had good reasons for their decision in that matter.
