They don't tell viewers what the charities are beforehand but it's not based on viewer voting so I don't know why. There has been controversy around the choice of winning charities, as it's a coincidence that they are both refugee based.
AI says:
The contestants competed for a wide variety of organizations. Winners Romesh Ranganathan and Sebastian Croft played for the Refugee Council and Choose Love, while other participants supported causes such as The Louis Dundas Centre (Amol Rajan), Bamboozle Theatre Company (Bella Ramsey), Friends of the Earth (James Acaster), Blue Marine Foundation (James Blunt), The King's Trust (Jerry Hall), UNICEF (Joanne McNally), and Tamworth Wellbeing and Cancer Support Centre (Joe Lycett).
Additional participating celebrities and their charities included Julie Hesmondhalgh (Reubens' Retreat), King Kenny (Rethink Mental Illness), Leigh-Anne Pinnock (British Red Cross), Maya Jama (Action for Children), Michael Sheen (Tŷ Hafan), Miranda Hart (Astriid), Myha'la (The Food Foundation), Professor Hannah Fry (Alzheimer's Society), Richard E. Grant (Longfield Community Hospice), Rob Beckett (Matt Palmer Trust), Ross Kemp (Haemophilia Society), and Sharon Rooney (Robin House Children's Hospice).