Open platforms often have individuals running/hosting their own repositories, which means the risk is distributed.
This means that the individual repository can be attacked without affecting the whole network. The risk is still there, but they would have to simultaneously attack all repositories at once and succeed with all of them.
In a corporate-hosted platform like Snaps, you have one centralized location that can be abused and that can affect all repositories in the system.
If someone hacks Canonical, they can make the whole Snap Store an attack vector without nearly as much effort.