Because "eisiau" doesn't cause a soft mutation but "ga i" does. I'm afraid it is confusing at first but if you stick at it, it starts to make sense. After a while you begin to see the patterns. It's not just the letter D that undergoes a soft mutation. Ga i gi? (May I have a dog), Dych chi eisiau ci? (Do you want a dog?). Ga i fara? (May I have bread?), Dych chi eisiau bara? (do you want bread?). Ga i win? (May I have wine?), Dych chi eisiau gwin? (Do you want wine?), etc.