I read something that went something like out of all the people who start martial arts, half will stop after their first class. Half of that will last two months, half of that will last 2 years, half of those will drop out before shodan, about another third after, and a fourth of the remaining group stay around long enough to show others the Way. Those are the sensei.
As for why... I guess it's just not what they were looking for... if they want it, they'll come back. If not, your dojo is better off without them.