Apart from all the reasons listed above
1) ALL TfL bus routes are flat fare, current fare £1.60
2) Reading Transport are one of the most innovative bus companies in the country, and with their expansion in recent years have taken over First's Green Line route 702 to central London, as well as local services in the Slough and Windsor area, with many to Heathrow Airport too. They would have a very goo case for complaining to the competition authority people in the exceedingly unlikely event of TfL muscling in, but it ain't gonna happen anyway.