The government has already been pursuing a 'one card to rule them all' solution for about a decade. It's called ITSO.
It's been designed by committee and is obsolete before it's even hit prime-time.
It's found some use for concessionary fares on buses, and, in theory will be rolled out across rail franchises - although the franchisees aren't terrible keen.
The DfT has paid TfL quite a bit of money to make the Oyster infrastructure compatible with ITSO. It's worth noting here that Oyster is firstly commercially successful and handles *way* more transactions than ITSO does - or is likely to.
The real killer reason why TfL won't be switching is said to be that the transaction time on an ITSO card it really long - far longer than for Oyster. Not so much touch your card, but hold it there on the reader for quite a long time. If everyone used ITSO on the underground, gate-lines would suddenly have enormous queues behind them and stations would close due to overcrowding.