The French specifically did not want to use that method, afraid of damaging the priceless artworks...............
Right the fire will destroy them much better
Artworks can be restored, and often have been, from water damage. Much of the art on the main floor of the cathedral is carved stone, anyway, which wouldnt be damaged.
My theory, for the little its worth (well, make that nothing), is that they suspected there was an accelerant, even something like phosphorous, and didnt want to spread it. The fire fans out almost immediately as if it is following something, perhaps something spread on the framework during the restoration work or planted at various points. But if it is terrorism, the French government is surely never going to admit it, so this will remain speculation.