Hi James, it is to make sure that the marshals are not out on the course for too long, to give some certainty over likely timings of runners. Kipper's suggestions that it 'ups the stakes' is probably true. There's no overall time limit: if you get to the Tower in 70 minute, you will be very likely to finish the course in under 6 hours.
I reckon that 95% of runners who attempt the race will have no problem at all with the cut-off. If you run regularly in this area, I am sure that you will be fine!