If you would like to read through how Blizzards MMR system works, here you go: patents dot google dot com/patent/US10857468B2/en
Some key spots:
“In another example, if a player has been performing poorly (e.g., getting killed at a rate higher than the player’s historical rate), the scoring engine may dynamically adjust one or more coefficients to match the player in a game that will improve the player’s performance. For example, the player may be matched with easier opponents, matched with teammates that historically resulted in better match outcomes, and/or placed in a game that is more tailored to the player’s preferences (e.g., players that play in games more closely aligned with their preferences tend to perform better).”
Summary: If you’re playing poorly, the Match Maker will pit you against others you will likely win against to keep you playing. Obviously, there’s a pool of people that would fall into this category and the MM would pair them together, causing our steamrolling issue.
This one is the most nefarious IMO: " The matchmaking system and method may be leveraged in various contexts as well, such as to influence game-related purchases, recommend a composition of groups of players, train or identify non-player characters (NPCs) that should be used, and/or other contexts.
For example, in one implementation, the system may include a microtransaction engine that arranges matches to influence game-related purchases. For instance, the microtransaction engine may match a more expert/marquee player with a junior player to encourage the junior player to make game-related purchases of items possessed/used by the marquee player. A junior player may wish to emulate the marquee player by obtaining weapons or other items used by the marquee player."
TL;DR: The MM will pair people together to influence purchases. IE if I have a legacy skin that is for sale, the MM may put me into a group with others who also play that hero to influence them to buy the skin(s).
Take with this info what you will.