TfL take the philosophy that you have to spend money to make money. Spend a modest amount cleaning trains and stations and more people will feel safe and comfortable using them. And given the record on every line they've taken over so far that's been proven correct.
As I said earlier, Southeastern and other some TOCs don't seem to grasp this, thus the trains and stations are sometimes disgusting and feel unsafe, and this puts off some from travelling. First impressions count, and the state of too many services mean repeat business is not happening, unlike on LO.
It's not the only factor of course, and there's lots of other things at play like the DfT not specifying cleaning standards or short term franchises, but still, too many TOCs havn't bothered even with 6/8 years left. They lack the nous and imagination it seems to work out a better environment equals more passengers. TfL do, and take the risk to reap the rewards.