Because it's easier for the school and the district. Go ahead and suspend everyone, don't assign blame, and wash their hands of the matter.
Here's a lesson you might not have learned yet, but I'll clue you in: Nobody wants to take responsibility. We are taught from the earliest age to own up to our mistakes, but that's just lip service for a higher ideal that we don't wish to aspire to, because it's easier to just deny responsibility and avoid being liable.
The school doesn't want a protracted issue because a couple of kids decided to throw some punches. Rather than do anything useful, like sussing out who instigated the fight, or assigning all involved parties to some kind of community service at the school, it's easier to just suspend everyone, mark down the suspensions on the students' records, and let the bureaucracy take care of the issue if it emerges again (for instance, if one student has a tendency to get into fights, then their record will reflect that and the system can eventually remove them).
This is what you get with an increasingly litigious society and an endless appeals system where the people who bring forth frivolous lawsuits face no penalties for their vexatious litigation.