These aircraft already have an ELT
But it is permanently attached to the aircraft. So for a water landing, there may not be much time between the impact and fuselage (and ELT) submerging. Once under even a small depth of water, radio transmissions are useless.
Deployable ELTs are feasible, technically (already used in some military aircraft). But the commercial transport certification issues differ and make this a rather expensive technology. Particularly considering the number of crashes where a deployable ELT would make a difference in recovery efforts*. The news isn't all in on this incident, but it appears that the aircraft and recorders may be located in a relatively timely manner. A deployable ELT might have only saved a few days of search effort.
*AF447 is one example of where a deployable ELT may have been of great value. MH370 is another.