A toothed belt is used for the engine cambelt since the rotation of the engine and the cam must be kept in synchronisation. There is no requirement for synchronicity between the main pulley, water pump, alternator and power steering pump - so it is simpler to use a plain belt.
It would be possible to design a system that used a toothed belt, but his would add complexity, since if the belt were to stretch, the teeth would start jumping/slipping, resulting in rapid belt wear.
Ideally a toothed belt should be in contact with the pulley over at least 180 degrees, adding further complexity to the design; whereas a plain belt can cope with a little more than 90 degrees contact.
For some time now, Ford have fitted dual belts on their engines, such that should one fail, the engine will continue to operate normally.