If you care, here is the reason...

Let consider Boeing C-17, a Boeing military aircraft for transportation, not a fighter or bomber, it is equipped with JTIDS to maintain communication including point-to-point, ground to air, SATCOM, air-to-air and etc. All these communication system on board C-17 technically can be installed on any Boeing commercial aircrafts. In fact, Boeing P-8 Poseidon is modified Boeing 737, which is also equipped with JTIDS. So it is not that the aircraft is incapable of installing such system, the problem lies somewhere else. 

Boeing 777 is equipped with ACARS, through the ACARS pings, they can determine the possible fligh path, and narrow down the search area. ACARS system includes the components on the aircraft, geostationary sat, and ground stations. Every ground station needs to be certified to operate. These ground stations also need government licenses. ACARS is a 40 years old system, it send messages in telex format through VHF SATCOM to ground stations. To build one of these ground station in a country, the country has to first allow you to use the frequency, and that itself requires negotiation. This negotiation is between by the ACARS service providers and the countries. Without it, the airliners operating on those flight cannot turn the ACARS on. Today, ACARS is out dated, and the next generation ADS-B has been developed for AGES. The US government requires all airlines to adopt ADS-B for the major aircraft in 2020. I don't know the time frame, but I recall China issued the first ADS-B license to its ground station in 2012. It is still in integration & testing phase. The international bodies are extremely slow in adopting pretty much anything. So practically, you won't see ABS-B for quit some time.

The ABS-B transponder on the airplane are avaliable, and comparing to the ground stations and operating license, the transponders are dirt cheap. But just the transponder, it doesn't do you any good.