Probably several reasons, here are a few off the top of my head…
One big reason is that a powder in weightlessness would be rather difficult to clean if it spills. It may stick to surfaces due to static electricity, and may block the filters in the air conditioning system. It could get into sensitive equipment and peoples eyes and lungs. My own kitchen has huel drops and spilt powder everywhere already, it’d be much worse in zero g. Any residue from a spill not cleaned up would provide a breeding ground for bacteria.
There is the added danger that some powders, like flour, are highly explosive when suspended as a fine dust in the air, and afaik no one has tested huel for explosive properties. Wouldn’t be surprised if it is explosive, or flamable.
Regarding the cost of food, the only factor that costs is the weight. You can’t eat dry huel, so you still have to supply the water to be used with it. The fact that it is dehydrated doesn’t mean anything but a longer shelf life and a difficult recombining process (iss food is rehydrated under pressure in a sealed bag, which is squeezed and massaged into a paste.
Standard huel would also require a new blender machine and delivery system as the standard huel shaker won’t work in zero-g since you can’t pour or suck the huel out. It’d just glob together due to surface tension and stay in the shaker.
Also, although I’ve had no problems myself, I’ve read some critism towards the delivery company that ship huel. I doubt they deliver to orbit (its not mentioned on the website at least).