It's probably because the market for them is not as competitive as it is for smaller ones -- that, and the amount of additional materials required to make them, which increases.as the cube of the linear dimension change.
If you do not mind taking it slow, I have used the small Futaba servos sold by Parallax to move up to 20lbs on a level linear track. Just gear it down to a 80:1 ratio.
graffix
Posts: 389
Comments
-Phil
But smaller servos.
Buy from Honk Kong and cross your fingers:
http://www.ebay.com/itm/Pz-15128-30g-servo-rc-boat-airplane-Free-Shipping-/320799987094?pt=Radio_Control_Parts_Accessories&hash=item4ab12b5596#ht_3978wt_1068
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=16643