There is no incentive for someone in half epic half legendary to try and take on someone in full mythic that is fully upgraded. That's why they all quit and honestly I think that's fine. Players who always lose to me do the same thing, they just instantly quit out on me. And why shouldn't they, trying to fight a new legendary mech with ~1500 hp while they still have 1000 hp and half old mythic half new items?
If there is some reason to play the game out, people would. It's far better use of time (and morale) to just re-queue and try to find a more equal opponent.
So don't blame the players who quit on you, blame the system that puts someone who is completely outclassed by you in a game against you and gives them no hope to win and no reason to play out the inevitable defeat. If you are a full new mythic Mech that is fully black level upgraded, you should expect everyone who is not on your level to just instantly quit out on you.
I almost think this is by design, a token attempt to discouraging 'pay to win' behavior because no one wants to play against your mech.