This decision came as a surprise, but as this Tweet explains, unlicenced drivers (14K of them) are using the app from another driver, so essentially the person who is taking the ride is not being driven by person that they should be. Which means that major changes are needed.

My statement on TfL's Uber decision. pic.twitter.com/uMDjoIlqOW — Mayor of London (@MayorofLondon) November 25, 2019