You can't use the exact same type of rolling stock on all lines.
Each line has different passenger loadings, different physical attributes, different line speeds, different station layouts, different EVERYTHING!
The uniform plastic stuff coming up for the SSL will only be the second time that a uniform type of rolling stock has been attempted on LU - the first was the Standard Stock of the early 20th century, which saw use on most of the tube lines of the day (Central, Picc, Hampstead, Bakerloo)
Would you want to ride a C stock train out to Amersham, or a 1973TS round the Circle Line?
Why (apart from the obvious tunnels) have different lines got different rolling stock. Wouldn't it make more sense financially to have the same cars running on all lines?
No. You may notice that the various tube stocks are known by the approximate year they were first introduced. Each was of course built to the "best" standards of the time. Standardisation would either eliminate development over time, or it would mean you would have to buy and replace a hell of a lot of expensive trains all at once (which doesn't sound like financial sense to me). Also, think what would have happened if all the lines had got 83 Stock and not just the Fleet - at least the disasters on wheels were relatively contained.
As for the SSLs, I suspect that standardisation would be possible, if the standard interior were comparable to that of A Stock (or even, shock horror, like the Southern's 455s). Kill the bloody Circle, and you have a real railway there.