The Canberra sits at a 'sweet spot' from the point of view of size, performance and complexity for the NASA mission.

The canberra was orginally designed for tactical bombing and reconnaissance - which means it had a good balance of speed, range, payload and stable handling properties; all of which are applicable to the type of data gathering research missions NASA are using them for.

In addition, being developed when it was the Canberra is much more of a "traditional aeroplane" instead of the much more complicated "integrated weapons systems" that later designs would become.

As aircraft developed through the cold war they would become more and more complicated and integrated with weapons and sensor systems - great for combat performance but not necessarily for NASA if these are not needed for their missions yet must be maintained to some degree to allow the aircraft to operate.

Also as the capability of weapons and sensors increased the tactical bombing and reconnaissance missions have been transferred more and more to multirole fighter aircraft - advance weapons and sensors no longer need the stability of an air frame like the canberra, and fighter aircraft have got big enough to carry the required payloads.

The payloads have also got smaller such that pure reconnaissance aircraft do not carry as much payload. The most comparable aircraft are probably the u-2 and global hawk, both of which NASA operate but dont have quite the same operating capabilities, mainly in terms of payload, as the canberra. Not better or worse necessarily, just not the same.

So in conclusion: the trends in development of aircraft means that very few aircraft have been developed since that have the same intersection of size, performance, handling and operational complexity as the Canberra. NASA do operate some very modern aircraft but none of them are able to quite match the specific requirements of the missions the Canberras fly as well as the Canberra can, therefore they continue to operate them for those missions.

Sources - I haven't found any technical sources (even this NASA page doesnt go into much detail) but this BBC article is interesting. Also the Omega Tau Podcast has a great series on flight research at NASA, although they dont talk about the canberra I think.