Realistically, to prevent manipulation of your MMR. Back in the day, it was discovered that players could exploit rank by playing non-meta heroes and gain rank even if they did not perform very well, gaining SR with winrates as low as 35%. Now the rules of the matchmaking and MMR have changed since then. Also the matchmaker now pairs teams more loosely since the introduction of Role Queue too. In the end the four basic rules applies:
- Wins will gain SR, Losses will lose SR.
- Your personal MMR is compared to the team average MMR of your opponents (this is called the “strength of schedule”)
- Your MMR will fluxate more rapidly the less frequently you play, the more you play, the more stable it will be. (Especially with new players.)
- For Platinum and lower players, there is a minor factor of “performance based SR adjustments” but it is very minor and only a real influence if you can really outperform with a specific hero. This adjustment is based on your performance with that hero in comparison to other players around your rank using the same hero (in different matches).