Lots of reasons given for why not, but in the past, one crash would instigate major changes, irrespective of cost.
ie TCas etc was brought in even tho very few planes ever had mid air collisions.
Saying it works well now isnt a help when you cant find the black box ( like MH370 ) Reports today quote we have spent nearly 100million so far.
Maybe to reduce costs we can use reverse logic and remove black boxes and all their extra cost, as not many planes crash these days.
Andrew