Short answer: money. Public institutions like the National Gallery need high-level sponsorship. The gallery explains: “Since 2008 Credit Suisse’s partnership with the National Gallery has enabled us to plan and create outstanding exhibitions showcasing the history of European art in new and surprising ways that have captured the imagination of over 1.6 million visitors.”
Credit Suisse has also funded a range of charitable projects at the gallery: “These have included a series of photography workshops for young people with Centrepoint, as well as workshops with the Alzheimer’s Society, The Place2Be, The Children’s Trust, and Credit Suisse partner schools.”