Answer:    

Among the reasons are other CAD LISP apps commands or utilities that DraftSight does not support.

     E.G.,   

A Visual LISP ® modification of vl-filename-base proprietary  function. This function returns the name of a file, after stripping out the directory path and extension. Here is the modification LISP code that allows that LISP functionality to work with DraftSight:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;(setq fn (vl-filename-base (getvar "DWGNAME")))

    (setq fn (getvar "DWGNAME"))

    (setq n1 (strlen l1))

    (setq n1 (- n1 4))

    (setq fn (substr fn 1 n1) )

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Note: Only Premium DraftSight, but not the Standard, can use LISP programs and commands.

To learn more about DraftSight LISP in general and regarding to the above,  go to this  blog:

 LISP Webinar No. 3 (It includes links to DraftSight LISP webinars No. 2 and 1 )

 https://swym.3ds.com/#post:15937  

Best Regards,