BBC: I'm sorry, I'm afraid that we're going to have to postpone the start of your season again. Budget concerns, you know. You'll just have do make do with the back half of Season Seven and a couple of specials for your 50th anniversary.
Doctor Who Producer: But our series is a phenomenal success! It's getting astounding ratings and great audience appreciation numbers every year, year in and year out!
BBC: Well, yes, but you see it's a question of money. Doctor Who is quite expensive to make, you see, and we've got a limited budget.
Doctor Who: But when you factor in overseas sales, merchandising, spin-offs, tie-ins, and the DVD market, the show is actually the most profitable thing the BBC makes!
BBC: AHEM. We do not sully our hands with mere commerce. We are the BBC, proud guardians of the glorious cultural heritage of Great Britain! It doesn't matter how much grubby, filthy lucre your series makes--we don't even keep track of such things!* What's important is the artistic merit of what you create, dear boy.
Doctor Who: Well, our AI numbers haven't dipped below the mid-80s since the series started. By the BBC's own standards, we're making a series that everyone who watches considers to be some of the finest material on television.
BBC: Yes, and that's wonderful. But you see, it's very expensive...
(wash, rinse, repeat...)
*Not literally true, but more or less accurate; money that 'Doctor Who' merchandising doesn't go back to making more 'Doctor Who', it goes back to the BBC's general Dramatic Series pool. They might keep track of it, but it doesn't count in the series' favor, because the BBC is not supposed to be concerned with turning a profit.