I understand the case of Rose Red, because there was already development going.
But while a theatrical or tv-film is often around 90 up to 120 minutes, a mini-series is much longer. You can write a filmscript knowing it will be in those time limits, but with a mini-series you will have to know how much time you have and how many episodes - to build in finales/cliffhangers on episodes for example. It can be 3 hours or 6 or other lengths.
When he writes Storm on the Century or Golden Years he can't do that without knowing how long it will be and how many episodes, because it will be useless if the tv company can't fit it in their programming. So, he must have some green light from the tv studios first to know how to construct the script, I can only assume. It appears to me they can only be written when he already has a deal with the tv studio/channel/network...