For beginners, Tefusang(ling)s may be way too hard to survive, because they are often quite well armored already. To be true, I have no character who is able to kill an adult Tefusang, after playing more than 6 years now. It was never necessary to level that much, now it is.
For the more experienced players, Tefusang(ling)s may have had too little value, so they already went for other opponents like Maulberlords. Who knows. Ask the "hunters" out there. The best reason was probably getting lard for baking...
But now that leatherworking becomes famous, I could well imagine that hunters of guilds may start on Tefusangs again. It may just take a little more patience...