Weight mostly. I used a blanket when horse camping as a kid. Weight was not an issue as the horse carried it. But it was heavy!

The standard for us was a very thick wool blanket and a canvas tarp. We wrapped up in them so that the canvas projected over our heads (it was quite a stiff canvas). Sometimes we tied the part over heads to something to keep it up off of our face. Did not seem to remember doing anything about bugs, but this was in the midwest so bugs may not have been an issue.

Very warm and versitile system and I spent many a night looking at the stars. Blanket and tarp were both multi-use items. When ever we stopped for anything we spread the tarp on the ground. Made for a nice sitting spot and we lost much less stuff when we used the tarp as a ground cloth. Small things just could not burrow down in the dirt. Actually, I still do that, but now I use a space blanket. They do not last as long, but much lighter.

Now I carry two space blankets and a bivy sack when hiking and NOT planning an overnight. OK for mild weather like here in Florida. Marginal for desert use though.