Stereos are limited in their resolution so they are limited in their magnification. Stereos are the most used microscope, they are found every were from ecology studies to machine tools. The binocular compound is the microscope used when high resolution is needed and when depth perception is not needed. Compound means that there are two major optical parts to the microscope, an objective and an eyepiece, providing magnification. The modern compound microscope is binocular.
Monocular microscopes are a commercial non-entity.
If they are sold at all they are sold to small schools that should know better. Monoculars are very hard to use even for experienced users. They take all the fun out of the micro world. If you have budget constraints and are thinking of a monocular microscope try buying good used binocular microscopes. This is a far better plan than killing some ones love of the micro world with an impossible to use microscope.