Project director Naoki Hamaguchi spoke about this in an interview. According to him, there was simply no other option.
It wasn't so much about “wanting” to make a trilogy — it's more accurate to say that, when objectively assessing the volume of the story and content that needed to be shown, no other option besides a trilogy was realistic.
Hamaguchi noted that he understood the scale of the task from the very beginning. This was especially true for the Midgar section — in the original, it is short but extremely rich in events, characters, and lore.
The Midgar segment in Final Fantasy 7 is a relatively small part of the original game, but it is incredibly dense with information about the world, characters, and plot. It became clear that recreating it with modern methods would require a very large volume of content for it to work as a standalone game.
That is why the first part of the remake takes place in Midgar and further explores the city, its inhabitants, and the social inequality within the conflict with the Shinra corporation.
A separate challenge was dividing the story into parts. Formally, the three discs of the original could have been turned into three games, but the plot required a more subtle approach.
According to Hamaguchi, Tetsuya Nomura had a similar idea, and as a result, the structure of the entire trilogy was approved quite quickly.