It costs a lot of money.
Planes rarely go missing like this.
It's really just a matter of economics. You'd have to shell out $100 grand per plane (some possibly mis-heard quote from NPR weeks back so take this number with large grains of sale), and it would cost a lot more upgrading infrastructures around the world to work with the new systems.
Then factor in the idea that some countries may not want to put in some better security/tracking of planes in the light of NSA reveals - let alone an american company spearheading this conversion like GM or Boeing; you start to see that for the most part people are willing to put up with a few missing planes a decade (awful, isn't it?) in order to save a couple bucks this quarter.