How to use the "cmi.progress_measure" to track the progress in a course ?

Good Afternoon, Eric.

You are correct in observing that although cmi.progress_measure is part of the SCORM 2004 Run-Time Reference, cmi.progress_measure is not among the information that Storyline communicates to Learning Management Systems.  To attempt to address your questions individually:

• Why questions are often among the hardest to answer, since it probably wouldn't be productive for our software architects to be active daily participants in our Discussion Forums.  Having said that, I can hazard a guess that the decision not to send cmi.progress_measure might have something to do with the non-linear potential of Storyline content.
• Here are the SCORM 2004 Conformance Requirements.  I don't see a requirement that the SCO must implement the cmi.progress_measure data model element, but please feel free to let me know if I've missed something there.
• Here is an excellent Forum Thread on building a custom progress bar in Storyline.  Once you have the progress information tracked in a variable, here is how you may potentially be able to send that variable to your LMS.
• Although Articulate doesn't provide support for JavaScript coding, this article will help when you're developing JavaScript triggers in Storyline.  ...and if you're looking for help from the Community, Steve Flowers is an excellent resource on all things JavaScript.

Good luck with your project!