The reason was no more OZ class 1981 and no more factory OMC racing.
Ken Stevenson/Bob Spalding won the 1980 Paris 6 Hour race driving the #12 OZ class Velden/Carlsberg/Johnson RS V6 3 liter looper.
First picture below.
OMC came out with the new V8 for the OZ class 1981 and Mercury pulled out of the OZ class.
The Paris 6 hour race organization made the decision to ban the OZ class ..(over 2 liter) with help from Mercury.
Only ON max 2 liter was now allowed in the 1981 Paris race.
Reality was that the OMC factory could not compete with the powerful 3.5 liter V8 in the race for the overall win and pulled out.
Last year for a factory feud between OMC and Merc was 1980
1984 OMC and a French boat race organization came together to run a race for the OZ F1-V8 class in Paris. This time with the pit in the famous Bois de Boulogne . Still racing on river Seine but far away from the famous Eiffel tower
I raced there with my #8 Burgess/Aspen/Evinrude V8 but not a great place.
Without a factory feud between OMC and Merc the famous Paris 6 hour race was no more
I know some people having a different opinion but I think this story is 100% right.

The last Paris 6 Hour race was run 1988..with very few..(if any) famous drivers and no factory teams.

The 1981 Race program/entry-list show few 2 liter ON class entry’s and most of the famous boat racers names are missing.