If you read this, know that I did not look any of this up and it may not be cannon.
my belief is that the mono-eye of the zaku is an extremely acute visual and infrared sensor with a limited field of view. this makes it very useful for long range engagements and for picking out the details of certain objects at closer range. these could also be linked to the scopes used by the pilots during long range engagement and reconnaissance.
Maybe these sensors were very expensive and difficult to produce, especially in the space colonies. thus, it made sense to have it swivel around and be able to select what it focused on. When finding objects, the eye swivels around acquiring large amounts of long range data to be processed into something the pilot can look at.
from a war perspective, it makes sense to have a minimal quantity of expensive and rare parts on a zaku. especially if it is not necessary.
The gundams having two eyes does not mean that they have two cameras. they may have more than two hiding behind the eyes in the head, thus allowing for a much larger amount of data acquisition and finding capabilities. maybe it is not that difficult to make the cameras on earth with the vast amounts of materials and large swaths of land at hand. or maybe it was that the gundam has a compound eye that processed many lower quality images into a higher quality image, just like the hubble telescope. It would still be vastly expensive to produce the compound eye, but it is an alternative to the single, scope eye that the Principality of Zeon has chosen for their mobile suits.
These two methods of getting high quality images both would work great, it is just that Zeon's methods produce a quicker image and is easier to salvage when a battle is over. that is, if the zaku didn't detonate.
that, and the zaku eye is sexy. and the zaku was not originally designed to be sold as gunpla. only the gundam and the federations mechs were designed to be both cool and modelable. the zaku was the brainchild of utility and looks, not sellability.