Why is a origin exclusive such as ME3 never have bioware point sales?

AFAIK, there has never, ever, been a sale on BioWare Points.

The games themselves, and the versions that include all the DLC (e.g. the UE or the GotY edtion, etc.) yes. 

But the Points?  No

And I would never expect that there would be such a sale. 

If you like the game well enough to be interested in the DLC, you've obviously already bought the game iteself.

Why in the world should the publisher discount the add-ons? 

When we're at two/three/more years after release, this is the only way they can expect any real revenue.