A drone can be used for stills a n d video! 
What Shadowtail describes is due to the distance of the individual shots, the slower the flying speed, the closer the images. And although the shots should not be taken too far apart from one anoter, a certain distance is required for the 3D information to be extracted.
However, that should not affect the number of detected features. In fact, a low number is a very good inicator for blurry shot, especially amongst many more images with a significantly higher number of features. But that is also not neccessarily the problem here.
I’d be curious if you get any improvements and keep my fingers crossed!