Probably the usual sort of thing, creative differences or financial differences. Happens a lot but can stymie films for years before someone gives up or is bought out.
Shame. I’ll be honest, I’m not a huge fan of the first one but I actually really like 28 Weeks later, be interesting to see how 28 Months Later panned out. Obviously the virus should have burned out again as it did in England because everyone infected died of hunger/thirst, but obviously if the virus was mutated by being carried by the mother it might work very differently. Certainly some of the “zombies” in the second one seemed somewhat more aware of the world around them.