Maintenance + cost.
The same features which make them frangible mean they are abraded and weakened by the elements and jet blast. Plus it is a certainty that someone drives in them with a fire truck or similar and breaks them.
Then, "when they is broke" they are not easy to repair as one might say. The FAA AC No: 150/5220-22A summarises it nicely:
Repair. The EMAS must be designed for repair to a usable condition within 45 days of an overrun by the design aircraft at the design entrance speed. Note that this is a design requirement only. An EMAS bed damaged due to an incident (overrun/undershoot, etc.) must be repaired in a timely manner. The undamaged areas of the EMAS bed must be protected from further damage until the bed is repaired.
Not too many airports tolerate more than 6 or 7 hours out of service for the runway !!!!! At JFK after the overrun, it is reported that the damaged EMAS material was removed with front-end loaders and discarded. The aircraft was pulled out, and then new precast blocks were installed and finish coats applied. No comment as to how many weeks it took.
Finally, a quote from Juneau, Alaska. They have their own problems with climate, but the essence can be distilled out:
RESOLUTION OF THE JUNEAU INTERNATIONAL AIRPORT BOARD
A RESOLUTION DECLARING ENGINEERED MATERIAL ARRESTING SYSTEM (EMAS) AS AN UNACCEPTABLE AND AN UNWORKABLE ALTERNATIVE FOR MEETING THE FEDERAL AVIATION ADMINISTRATION’S (FAA) RUNWAY SAFETY AREA STANDARDS AT JUNEAU INTERNATIONAL AIRPORT.
RESOLUTION NO. 2005-01
WHEREAS, the technical information about Engineered Material Arresting System (EMAS) supplied by the FAA does not establish that the technology works in all circumstances, and WHEREAS, technical information supplied by the manufacturer of EMAS establishes that the material is still being analyzed by testing contractors, and there is no track record of the technology's behavior in anything approaching a temperate rain forest similar to Juneau's climate, and
WHEREAS, after damage to EMAS due to an aircraft overrun, the runway safety area is compromised until repair can be accomplished, probably not a possibility in winter, and
WHEREAS, repair costs will be borne by the airport simultaneously with the costs of litigation to establish the responsible party, and
WHEREAS, frequent inspections of over five miles (one end) or over 10 miles (two ends) of caulked seams are required to maintain watertight integrity of the installed system, and
WHEREAS, snow and ice removal with special equipment would be required, and
WHEREAS, the cost of installing, maintaining and replacing EMAS is significantly more than the cost for a traditional extended runway safety area, substantially increasing capital and operational costs at the Juneau
International Airport, and
WHEREAS, these increased operational costs will ultimately have to be paid by the traveling public and the air carriers serving the public at the Juneau International Airport,
NOW, THEREFORE, the Airport Board of the Juneau International Airport, Juneau, Alaska, declares EMAS in its present form and at its present cost to be an unacceptable and unworkable alternative to runway safety requirements at the Juneau International Airport. The Airport Board declares that EMAS is neither a prudent nor feasible option for meeting the FAA's standard for runway safety areas at the Juneau International Airport.
The Airport Board of Juneau Alaska further declares that the installation of EMAS at the Juneau International Airport will unacceptably circumscribe the operating and maintenance budget unrelentingly for the foreseeable
years of the existence of EMAS and that consequently the Airport Board cannot commit any of its budget to either the installation or the maintenance of EMAS.
Passed this 8th day of June, 2005, in Juneau, Alaska.