The key phrase is "satellites separated from the final stage safely without colliding into each other"
Let's break that into two pieces:
Satellites Separated From Each other- In order to do this, it takes time, power, and attitude adjustment.Also, they have to be arranged such that all of the satellites can be separated from each other. Most of these were nano satellites, so they can be put together into a single dispenser that could contain at least 3 satellites, but there were still a considerable number of them. Still, that requires some work.
More interesting is the second part, without colliding into each other. There is something about separating satellites that isn't well considered. If you put a satellite ahead of you, then that increases it's speed, and it will actually orbit slightly slower, ultimately having it be at the end of the pack. Satellites released into a retrograde direction will have the opposite problem. Bottom line, they will have some period in the first orbit where they will be very close to each other, and with 100+ satellites, one has to ensure that no pair of them collides with the other, which isn't a small task.
The way that they accomplished this was to release most of the satellites in the same direction. The primary satellites were released in a different direction from the nanosats. This allowed them to separate successfully.
As a point of comparison, SpaceX, the last I heard, will only allow up to 12 satellites on the same launch, although that number might have increased since I last heard.
For more information, check this out.