I learned "plancher" for floor; wrongly marked wrong.
Apparently that only applies to wooden floors, so is not a generic term for floor.
The implication here is that the weather is too cold, not the floor.
I wonder if it accepts ...parce qu'il est trop froid if you mean that the floor is too cold.
Apparently it does.
Le sol ne sèche pas car il fait trop froid. Accepted :)
Can one use "car il" (instead of parce qu'il...)?
Yes, but note that there are some subtle differences between "car" and "parce que". See this thread for more details.