Any locator beacon fitted to the outside of the aircraft would need to be able to withstand temperature changes between +50C and -70C to cope with sitting on the ground in the tropics and flying at altitude in more temperate zones. It would need to withstand acceleration forces of up to 400g during a crash and reliably separate when needed, but never under normal operating conditions.
Such a system would need to be heavy and complex. Additionally power and or data lines would need to be routed through the pressure bulkheads adding additional stress and failure points to the aircraft fuselage.
By contrast, a GPS beacon system can be located within the fuselage with a patch antenna sited in one of the radio frequency transparent panels already mounted on aircraft. The environmental conditions are far more friendly to the electronics, so reliabilty will be better.
Even using off the shelf components costing a couple of hundred dollars, fitting such a device will probably come in at between US$50k and US$100k per aircraft considering engineering costs and lost revenue while the aircraft is being fitted out and tested. I can't see many airlines agreeing to that when existing systems can do the job, assuming they are not sabotaged by the flight crew.
MH370 is a particular case where deliberate efforts were made to hide the aircraft from surveillance. AF447 took a while to locate and recover because of the depth of water at the crash site, although wreckage was quickly found floating on the surface.
External, detatchable floating beacons are not the answer to the problem of locating downed aircraft at sea. If the crash is survivable, then a couple of emergency beacons packaged in the liferafts or emergency slides will suffice to alert rescue crews of the location of possible survivors.
If the aircraft is totally destroyed, then a 15 minute GPS breadcrumb trail along the flight path will narrow down the search area to a maximum of a 150 miles from the last recorded position. A far smaller area than is currently being searched for MH370.
There will always be the chance that an aircraft will totally disappear even when a rough estimate of its final position can be made. The aircraft could crash into mountainous terrain and be buried in an avalanche, or into a remote area of sea where weather conditions are so severe, that floating debris is destroyed or dispersed completely. Eventually something will turn up, but it may take many years of searching or waiting. Sometimes it is just not possible to obtain instant answers, no matter how distasteful this is to the Internet generation. The aviation world is intensely aware of cost management and no airline will fit any additional equipment beyond what is legislated for or will save the company money.