Workshop

A rather immodest question: why couldn't they have made it a 100% chance and avoided unnecessarily complicating the setup?

Because the double shot itself is a countermeasure against revealing the main roles, rather than simply helping the Mafia to quickly thin out the ranks of the town.
Therefore, 75% is implemented to make each side think about their actions. For the town – what if it doesn't reveal anything – what should they do then? For the Mafia, the opposite is true.