Charities Don't Believe in Charity

From the BBC:

Six Welsh charities have urged the UK Government to rethink proposed cuts to housing benefit.

...

"We urgently call on the government to reconsider the cuts, which will disproportionately affect those in greatest need, including homeless and vulnerable families, those in need of housing-related support, people with mental health problems and children living in poverty, and result in greater long-term economic and social costs for local authorities, housing associations and government in Wales."

If there is public support for keeping housing benefits unchanged why don't these charities simply ask people for the money voluntarily? Could it be that they don't think they'll get anything?