I think the biggest obstacle was an emergency exit on the lateral side, which is still a total construction site thanks to the second part of the parking garage, the one on the right side on the Camp Nou live stream. In the last 2 weeks, they rushed to build a bridge over that gap and installed stairs on both sides to get this emergency exit done. They even skipped the second floor of the parking and will do it later.
That said, the city council demands the entire area to be closed to allow people to walk there, which is understandable, so I think that's why they came up with these different phases. In 1A, lateral will still be closed to the public as the parking garage won't be ready yet, so only gol sur and tribuna will be open to the public. Then when the parking garage is completely finished and the gap on the lateral side closed, 1B will start and in the end, when goal norte is finished 1C with 60k fans.
That's what my logic tells me, without reading the plan behind these phases.