The answer to this question would be: d had no natural predators on the island of Mauritius.
To answer the question, you just need to read the article in passage 1. This was written there:
When the dodo's ancestor (thought to be a migratory pigeon) settled on this island with abundant food, no competition from terrestrial mammals, and no predators, it could survive without flying, and thus was freed from the energetic and size constraints of flight.
The reason should be because there is no predator there so dodo doesn't need to fly.
