It's just that you have to wait a few months to know if the vaccine you're testing is effective. You can't make this delay shorter with more volunteers.
And when you're fighting a decease that "only" kills 0.5% of the people inflected, your risk margin is pretty low (what if your vaccine creates deadly consequences to 0.6% of the people vaccinated?).
The mRNA vaccines had to wait until November to get enough infections.
There can be an issue that the measurement of the effectiveness of the vaccine is then related to the exposure protocol (which may not be the same as typical natural infections), but it's reasonable to expect results much sooner.
The challenge arm wouldn't have any influence over the other arm (and likely not much impact on recruitment), but might provide results for some groups much faster. Starting vaccinations on younger healthcare workers in August seems like it would have been a win (assuming they had data to justify it by that point).
Challenge trials need far fewer people, so paying each one $1M is feasible.