Size: these mirrors can only be made in hand-held size. Anything larger requires exponentially more energy input while creating them. Or the glass of the mirror needs to be thicker until it becomes infeasible, etc.

Magic: these mirrors were made by people using magic, maybe they can only be used by people who can use magic. Pavel was lucky that he actually can use magic but he belongs to the >1% of humanity who has the ability encoded in their genes.

Connection issues: it's not a coincidence there were originally only two mirrors: there is nothing in the magic that binds two of these together, they simply find the nearest magic mirror and connect to it. Or, alternatively, all of the magic mirrors within a range are connected at the same time.

Size: these mirrors can only be made in hand-held size. Anything larger requires exponentially more energy input while creating them, or the glass of the mirror needs to be thicker until it becomes infeasible, etc.

Magic: these mirrors were made by people using magic; maybe they can only be used by people who can use magic. Pavel was lucky that he actually can use magic, but he belongs to the <1% of humanity who has the ability encoded in their genes.

Connection issues: it's not a coincidence there were originally only two mirrors: there is nothing in the magic that binds two of these together; they simply find the nearest magic mirror and connect to it. Or, alternatively, all of the magic mirrors within a range are connected at the same time.