Years ago in an accident like this the stock involved would be mostly knackered and unable to be used again. You might try and salvage the locos, but the rest could be happily broken up on site, maybe even burnt. Not there's a good chance most, perhaps all, will be reusable, so you want to recover them. I understand initial estimates were that a small number were unable to be moved by rail, and would need craning off to be moved by road, mid-week opening might have been possible, that changed as more thorough examination was possible and now most will need that doing. That can only be safely done, while preserving possible evidence also, during daylight hours and even with the long days moving 3 per day is a realistic max, providing you've enough trailers to even move them away, so you're looking at perhaps 4 days for that, then you've got the overheads to take down and string back up, any track and signalling repairs to carry out, and that's all even beyond RAIB investigations being finished to allow that.
Now, maybe some service could be partially restored on Friday, but even if it could, and maybe even was, do you want to publicise that in case timescales slip? And is it even worth it, opening an an undetermined time of Friday only to close it again for another blockade at the weekend which might be affected by the final works? Might as well get it all done in one go, leaving the PW free to try catch up on some of the engineering opportunities no doubt postponed from this weekend.