Because of the danger from flash and because it helped the pro photographers. They used to say that a flash from the centre green didn't affect the riders as much as from the crowd. Having said that, you still saw people doing it but there would be a "no unauthorised photography" notice somewhere in the programme, as well as a "betting prohibited" notice. These days you can get a bet on some meetings, so I suppose the 'no betting' sign has gone, I don't know whether any clubs still have the 'no photography' notice either.