Another point to add to CapnZapp argument:
Balancing monsters
If 5 ongoing stacks with other ongoing, it's hard to measure how much output damage the monster has.
5 ongoing damage is roughly equivalent to 9 damage.
Example: 2d4 + 15 damage would be equivalent to 2d6 + 4 + 5 ongoing damage
Now, if you add the "don't stack" rule, suddenly this equivalence is no longer true.
Balancing monsters
If 5 ongoing stacks with other ongoing, it's hard to measure how much output damage the monster has.
5 ongoing damage is roughly equivalent to 9 damage.
Example: 2d4 + 15 damage would be equivalent to 2d6 + 4 + 5 ongoing damage
Now, if you add the "don't stack" rule, suddenly this equivalence is no longer true.