It all boils down to risk analysis vs. investment required. For example, CFIT accidents were rather common until some 20 years ago, so the development of (E)GPWS was reasonable and appropriate, the costs of it were not prohibitive and outweighed by the gains. The same for TCAS/ACAS (however You want to call it).
Please forgive my doubts, but seeing that serious volcanic eruptions are few and far between (and VA related incidents luckily even more so) and also seeing the airlines reluctance in parting with their scarce money I cannot imagine having such a system in widespread use or even compulsory like the two previously mentioned ones in the foreseeable future.