Because there's no reason why that episode wouldn't have worked just as well with Ecclestone in the Hurt role - obviously with a few tweaks, but I can imagine Ecclestone essentially fitting in as well. You'd lose the whole "We don't call him The Doctor" thing, but then that only turned up in the final 30 seconds of The Name Of The Doctor anyway - so as long as they knew before they shot _that_ that they weren't getting Ecclestone they could easily set things up for Hurt instead.
The three-way conversations between Tennant, Smith, and Hurt would have seemed a lot more crowded with a fourth character as well, I'd have thought. And they'd have had to have some setup to get him involved too. So if Ecclestone had been in there they'd either have to rework the episode significantly, or not have him as part of the main thrust of the episode (possibly just bringing him in for a more explicit scene at the end?)
Sadly, I don't think we're likely to know - Moffat certainly isn't talking about it.
Original post on Dreamwidth - there are comments there.