To turn Tropical Storms into Hurricanes, you need: water temperatures at or above 80 degrees, a lack of wind shear (change of winds with height), a moist air environment, and open water.
Tropical Storm Iasias has plenty of warm water, and low wind shear. However, there is storm inhibiting Saharan Dust in the forecast path of it.
Furthermore, the storm has through travel through the Dominican Republic, who's mountain terrain will help to break up the storm. Therefore, the storm is expected to remain at Tropical Storm strength.