It's weight-saving planning. When grams (or ouncey things) matter, why install a device with a measurable weight to manage a potential problem which can be...er...handled procedurally (don't shut the hatch on the way out).
Acknowledging that I am no engineer, and am basing this entirely on lay observation, was there even a mechanical issue to be solved? Or even, was the mechanical issue actually
solved by not having a handle on the outside?
We are used to a world where most doors latch automatically when they close, so you have only to pull or push them closed and *click*, you can't get the door open without operating a handle. Hence people shut themselves out of their houses or cars. The lunar module hatch 'handle', as far as I can see, has a locked and an unlocked position. In other words, it only locks or unlocks the hatch, it doesn't disengage a sprung latch that holds the hatch closed when you push it all the way home. If there is no sprung latch involved then it won't matter if the hatch is closed all the way, you'd still be able to open it just by pushing it open from the outside provided the handle is in the 'unlocked' position. Since it can't be moved to the locked position from the outside, there is no problem.
Without a handle on the outside it a) is not possible for the astronaut to close the hatch all the way (you can't pull it flush with the frame if you have to have your fingers hooked around the edge of the hatch to pull it closed from the outside), and b) avoids the issue of the hatch being closed and locked from the outside and then the handle jamming or breaking.
To me it seems that having no exterior handle on the hatch is actually the
best option as it in fact renders it impossible for the astronauts to accidentally shut themselves out of the LM while on the lunar surface.