I think one of the reasons are that they want every bit of light gathering ability available to them and don't want to compromise. If you want a lightweight rifle and hunt whitetail very early or late you probably need at least a 40mm objective and as good glass as you can find and yet have a lightweight scope. Not many scopes qualify in this regard except certain Leupold fixed powers with 42mm objectives, Swaroski a Series which may be one of the greater compromises out there..42mm objective...great glass and light and compact. Certain Kahles would also qualify in this regard. The new simmons Aetec scopes also have great dimensions.