llondel,
It appears to me that the primary cause of the accident was the failure of ATS to separate the aircraft. There will be many contributing factors to that, including communications coverage, the design of the radar system, and the display of same system, that presents flight planned level as opposed to last cleared level when mode c isn't available, triangulates the altitude crudely using a form of slant radar, and, when the mode c drops out, presents a tiny wee block of info to the controller concerning this. There also appear to be fairly widespread systemic problems within the structure and administration of the organisation.
The TCAS, as has been correctly pointed out, is a last-ditch, last chance technology, relatively new, designed to prevent a collision should the ATS fail, or the crew are at the incorrect level.
It's failure to work and the reasons for that, while extremely significant, should, I believe, be listed as contributory factors.