Both Conventions were silly, the 'flu was not entirely over then. If you look at the figures, it appears they did not lose too many, the big Exodus took place after 1925 , when Rutherford's nonsense about that year became obvious as the ramblings of an alcoholic.
This caused many, who were loyal to Russell and his teaching, to leave, and continue on their own. Some remnants of this group remain active today in the USA and in the U.K, and are still known as Bible Students.