A possible explanation, also from the ambergris Wikipedia page:

> Ambergris used to be very highly valued by perfumers as a fixative that allowed the scent to last much longer, but it has been replaced by synthetic ambroxan.

But buyers still exist for the natural stuff.

> A 1.1-kilogram (2.4 lb) lump of ambergris, found on a beach at Anglesey, Wales, was sold to a French buyer for £11,000 at an auction in Macclesfield, England, on 25 September 2015.

I think it has more to do with demand. Ambergris can't be found in large quantities. This makes it useless for big houses like Chanel,Dior,Guerlains etc. The other thing is consistency. The commercial fragrances want their fragrance to smell the same every year. Something you cannot guarantee if you do not know what Ambergris will you stumble upon. These days Ambergris is used mostly by artisan perfumers.

>> Just have to stumble upon it.

You can physically remove it from the whales, which is why ambergris trade is illegal in many countries. Even where legal, you have to be able to prove that it was pooped out and found, that you didn't kill a whale to get it.