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

Probably never happen. None of the DLC for any of the other BW games have come down in price (Dragon Age: Origins, ME1, ME2, etc..) nor have I ever seen points on sale, even prior to BW being assimilated.

The DLC costs more than the base game at this point, but no one is going to buy it at those prices.

At least Dragon Age has an Ultimate Edition which contains all the DLC and the expansion. I found that for €5 once.

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.