A thank you is more than just an automatic politeness.
A thank you says, I see you giving extra effort. Or a thank you says, I understand the work you have put into this. Or a thank you says, I acknowledge you. Or a thank you says, I appreciate what you have done.
The staff, board members, volunteers, exhibitors and speakers are all working on behalf of the association. Many are doing more than what is expected. When there’s no real feedback, no appreciation, no thank you, not only will they not only not want to do more than is expected, soon they won’t want to fill those roles at all.