Why is it recommended that people with glaucoma don't take midol?

Narrow angle. Only certain untreated narrow angle glaucoma patients should be extra cautious with midol or any otc medications with antihistamines, decongestants or other ingredients that could partial dilate the pupil and induce an angle closure glaucoma attack. Ask your doctor if you are at risk.
For angle closure. Midol can cause dilation of the pupil and can initiate an acute attack in patients with narrow angles they should not take any medicine that can dialate the pupil.