Treated fabrics are generally made of cotton or a cotton rich mixture, with polyester or nylon as the added yarn.
Treated fabrics are generally the cheapest form of flame retardant material but this does not mean that it has a lower performance than the other types.
The flame retardancy is achieved by adding a finish or impregnating a chemical polymer into the core of the material. Some finishes can wash out over time but the impregnated polymer types will last the life of the garment.... at least 100 washes.
As a fabric, the comfort of cotton always takes some beating. It is also a widely available fabric and from a renewable source.
The two major draw backs to cotton are its high level of shrinkage and its durability. The cotton rich mixtures will attempt to counteract these disadvantages, with varying degrees of success dependant upon the type of yarn added.