Tailor’s clappers are made out of hardwood. The wood has to be heavy and close-grained in order to do the job perfectly. The weight matters as well as the close-grain wood. If the clapper is to light the heat will be absorbed too quickly and ultimately your seams would not be as flat as you wish. If the tailor's clapper is made out of less dense wood, it can absorb the steam or humidity that stems from ironing which will result in breaking the tool.
This is the reason why Maple is one of the most popular woods for a tailor's clapper.