why is the data not continually sent to an earth based recording device via radio/satellite?
Because someone needs to pay for all that expensive satellite bandwidth, and when that someone has a large fleet of aircraft, the cost soon becomes prohibitive.
Also why waste bandwidth continuously transmitting and storing stuff that you're not really interested in (the whole point of the black box being you're interested in the data running up to an "event").
Basically you'd be looking at a whole redesign of the Blackbox system (as well as perhaps come co-operation from satellite providers on pricing). Given the relatively small number of hull losses, and the even smaller number of hull losses where blackboxes are not recoverable (either due to location, politics or otherwise) ... you can see why a major redesign might not be high on the manfacturers priority list because they know their customers probably won't be falling over themselves to pay more for the newer kit (let alone go back and replace kit on their existing fleet).
One day it'll happen... but you have to understand its not something anybody can wave a wand and make happen overnight.