It's also because the only way you know your vaccine works is if more people in the placebo group get the disease than in the vaccine group, for the appropriate

It's also because the only way you know your vaccine works is if more people in the placebo group get the disease than in the vaccine group, for the appropriate measure of statistical significance. If you test two effective vaccines against each other, you have a much harder time knowing if they were effective because you can't measure the cases that were avoided in a reliable manner, so it takes much longer to get a reliable positive result.