For each group that has declined it is for a different reason. Some have said that they don't have any studies or information or research that would provide any insight to committee members. Some have said that on the issue on which they were previously lobbying they no longer feel they have to lobby. I've had a witness decline, specifically for this study, due to scheduling conflicts. When a witness declines because of time, that's another thing.
It's for various reasons. If members are interested to know specifically which groups, you can always contact the clerk's office and I'd be happy to relay that information to you.