This may come as a surprise to you, but there are organizations, for example German municipalities, that have more than 500 users but can't afford to start pumping tens or hundreds of thousands per year into a file sharing service. Nextcloud themselves recognize this and offer 95%+ discounts to edu, similar to what Adobe, Microsoft, and Git[Hub,Lab] are doing.
How dare they. I just want to share photos and calendar with the 502 people in my immediate family.