(I haven't been following this case closely like others so this is just my assumption.)
I am guessing that there isn't enough info. to lead authorities to conclude that the Currier's are deceased so that's why there may not have been cadaver dog searches. Also, besides around the home area and where their vehicle was parked, where else would be searched? Doing cadaver work takes a fair amount of time and effort and without having any actual information or evidence on where to search, it isn't prudent to use certain resources. I would think that a cadaver dog would have worked the Currier's home, their vehicle, and where the vehicle was located, but I haven't seen this mentioned.
I am assuming where you mention "helicopter search with radar" that you mean a FLIR unit (forward looking infrared which is an imaging technology that senses infrared radiation). This would have had to have been used very early on. I am not sure the availabilty of this but I am sure that this can be used if again, there was information or evidence that the Currier's were in a particular area. Using "air resources" are extremely expensive and there appears to be no evidence of what happened to Mr. & Mrs. Currier, therefore the air resources are not prudent to use either.
If there were areas of interest then possibly the areas could be imaged by an imager in a helicopter (or in certain types of planes), and then the images could be analyzed. But again, there has to be evidence pointing to some place.
It must be very frustrating for the family, friends and people of Essex Junction, not knowing what happened and where the Currier's are (prayers for them all). Hopefully the police have information that at this time, they can't release that they are holding on to, waiting for information that will break this case. Let's hope so.