To ensure balance, there is a soft reset at the start of each season.
Why do we reset ranks? Why not just set everyone back to Bronze? We do this to keep similarly skilled players together in the same groups, and to protect new players from being beaten by veterans. This gives everyone a more fun experience overall - as well as giving everyone new goals to reach.
As you win Online Matches, you will be promoted to higher Divisions and Ranks during the season. Losing more Online Matches than you win will demote you to lower Divisions and Ranks. For more info, check out: What are Rocket League Sideswipe Competitive Ranks?