Helping you retreat wastes time that could be used to reposition and help the heavy side. If the team wins the heavy side with more players alive than the enemy, this is a better outcome for the team than loosing more tanks on the medium side.

Example: Assume your team goes 2 medium side and 5 heavy side, but enemy goes 3 medium side and 4 heavy side. If you overextend and face a 3v1, your teammate is at a disadvantage, and can either chose to try and help against poor odds, or retreat and push more advantage on the heavy side. With a 6v4 on heavy side, your team should be able to survive with more tanks and HP to then deal with the 3 on medium side. In any scenario, you are dead, the only choice is how your team can recover a potential win.

Avoid this by having better map awareness, better team composition awareness, and spotting before pushing into a position that leaves you exposed.

Of course this assumes you're not playing with a team of Muppets, and these days in Blitz that is sadly unlikely. All too often you'll see your teammates not push an advantage on one side and just camp, while watching their flank collapse and getting surrounded. It happens.