This is because of the way chunks are distributed (stored) on the Network.
If someone uploads (PUTS) a 100MB file. That file will be broken up into 100 chunks (1MB each). Each chunks goes to a different vault address in XOR space. This is done intentionally to decentralize the file itself.
So by having more addresses (vaults) you’re able to collect more chunks.
Off-Topic
I’m sure this is the next question, so I’ll answer it right now.
Why not just make all vaults 1MB?
The partial answer is the sigmoid curve reward ratio.
Quote from @dirvine
Sigmoid curve will allow very small nodes to get rewarded but encourage use up to average required storage. Otherwise we have all tiny vaults and that will be of no use to anyone. So sigmoid allows even small vaults to be rewarded but also encourages greater provision. A balance of reward verses requirement.
The other part of the answer is… hosting many 1MB vaults will be insanely resource intensive with a negative return of Safecoin farmed.