Lots of small charities, like Scout Groups, have perfectly serviceable and safe minibusses which will become illegal to drive in London soon. They do not have the resources to replace them and still provide the valuable services to the young people the support.
The amount of pollution that would be caused by these vehicles is very small (these charities only used their vehicles a small amount compared to businesses) so the downside of allowing the exemption is small, whereas the upside is massive.