I'm tempted to ask, what makes you think we can't?!?
But from my own professional experience, I can tell you that a failure to detect nukes is not caused by lagging sensor technology development, it is caused by the simple physics of the task. Beyond about 20 meters, even the Gamma Ray radiation from an unshielded nuke gets lost in the background noise from space. There are some real cute computer programs that use statistical analysis that can pull a signal out of the noise even when the signal to noise ratio is less than unity, but they need a long sensor signal integration time for that to work. That puts such detection methods outside what intel can make practical use of.
The problem with satellite, SR-72 and Aurora(?) photo intelligence is two-fold: First they will always have far more photos than gifted analysts and second, it's to easy to avoid operations when the satellite is within photo/sensor range. You can even go to a NASA website and get accurate satellite tracking data.
The technology to detect marijuana (and any other plant form) from an airborne platform is over 30 years old. The infrared sensors are so sophisticated they can tell you what species of plants/trees are being observed, how many, their maturity and even their health. But that technology is so expensive that it is rarely used on such frivolous pursuits as chasing down marijuana. That is usually accomplished (when airborne) by the human eye and from relatively low altitudes.
Regards,
Boot Hill
Because we aren't using it in Iraq are we? If we are, then there must not be any detected thus far. If we aren't, then why not?