Energy is proportional with the speed squared, momentum with the speed. You have a fixed budget of energy, initially stored as chemical energy in jet fuel. The engines have a more or less fixed thermodynamic efficiency, so you transform the chemical energy in a fixed amount of kinetic energy of the air being pushed out of the engine. You want that air to have as much momentum as possible (Newton’s action-reaction principle: the momentum the plane gives to the air is the momentum the air gives to the plane).

Now, for a given energy, you can push some mass of air with speed v or 4 times as much with speed v/2. The momentum of the air in the latter case is double the first case: 4 m v/2 = 2mv. You always want to move more air at lower speed.

You achieve that with higher and higher bypass ratios. Of course, with a higher bypass ratio the engine becomes larger and heavier (and draggier), so there are limits. But if you could increase the bypass ratio without increasing the engine weight and drag, you would always do it. As a bonus, with lower air speed come lower vibrations and noise.