GW likes to retain a hold and control over their IP which means that the only Hollywood director they'd work with is one who REALLY wants to make a Space Marine (or other) film. Because otherwise Hollywood would want to change and adjust everything.
Another aspect is that GW got burned very hard by Lord of the Rings. GW secured the rights for the models and made good money off them for years, but GW isn't a film toy company. They were not prepared for the very sudden and very big drop off in casual popularity that their LotR line had when the films ended. Big toy producers are used to this and know how to balance production as things ease off, plus they are already making and selling a new line of toys for a new film. GW wasn't and got hurt hard by the big drop off.
That in itself might make them cautious about a future film.
But overall I think its just that they'd want to retain control of things and "should" be big enough that their legal team would write up a good contract for them. A lot of Hollywood producers prefer preying on lesser known IP that gives them control over it.
