It does have a little to do with Oyster. The one day travelcard price is used as a cap for Oyster journeys. For people who use buses most often (and others) it is much more convenient to use an Oyster card than to buy a travelcard, the advantage is that your expendature may be less but never moer than the equivelent travelcard. However, if you needed to get a train you used to have to calculate which would be cheaper in the morning Oyster or travelcard, and then if your estimate is that you would save money by getting a travelcard, then you would need to find somebody to sell you one on your way to the bus stop.