The main reason is the limited range of practical use cases for low energy transfers.
The only low-energy transfers that can be executed in a manageable amount of time are those moving spacecraft between closely linked Lagrangian points, in particular the L1 and L2 points.
In our neighbourhood, these locations are the Earth-Moon EML1 and EML2 points, and the Earth-Sun SEL1 and SEL2 points. It's also possible to navigate between the EML points and SEL points, as well as to a temporary high altitude lunar orbit.
The problem is, those locations contain ...nothing. It's empty space. Even in the cases where they are suitable as locations for spacecraft like solar observatories, there's usually no need to move between them.
If you can reach one of those locations, you can reach them all through low-energy trajectories. But the main cost is still reaching them in the first place, which is approximately equal for all of them.
They are not usable for interplanetary travel.
In practice, they will only be used for "extended mission" type of objectives, where an existing spacecraft with very little propellant left and already in an L-point/high lunar orbit/high Earth orbit can be redirected to do a lunar flyby or escape into a near solar orbit.