I would say that the people who quit are the ones that wouldn't be very helpful if they did stay. Usually the more dedicated ones will stay in the team and will contribute a lot.
1. They quit because they are too lazy, too busy, not interested anymore, or have an attitude problem and got really mad at another team member.
2. Recruit the kind of people who will not quit
Try to keep it interesting. Building a robot,lloking through new parts, discussing strategy, etc. is much more fun than cleaning storerooms or doing fundraising.