The answer to both questions is probably just time. It took centuries of Roman rule for Latin to become the everyday language of the Western half of the Empire (the Eastern half kept speaking Greek). Also, the everyday language was vulgar Latin, not the classical Latin someone who studies Latin learns today. Classical Latin basically preserved the formal Latin of the Roman Republic, while the everyday spoken language had another five centuries to evolve before Rome fell and the individual dialects began to diverge into the modern Romance languages.