That was EXACTLY why he did it. He hated the Orléans clan (despite the duc de Nemours working for reconciliation between the two branches) and saw the Count of Montizon, his brother-in-law (Juan III) as his legal heir, not the comte de Paris.
He was willing to adopt the Prince Impérial as his heir for God sakes! (sound like a man who was as archreactionary to turn down a crown over a flag?)
He knew he had no children. That when he died France's throne would once more be considered open season between the
blancs (supporters of Montizon), the Orléanists (supporters of the comte de Paris) and the Bonaparte heir.
His INSISTENCE on the flag was a very loud, very clear "fuck you" to the Orléans clan (and ruining their chances of getting the throne in the process). Was he conservative and out of touch? Probably. But you have to admit that for someone who HATED who his successors were, and who didn't have a brilliant hand to play, he played it very well. It meant that the Orléans wouldn't get the throne but also that the FRENCH had declared against a monarchy. Not Chambord, therefore the Orléanses couldn't blame him (beyond being a seemingly sentimental old man).
Seriously though, the man spent more of his life outside than inside France and it was the July Monarchy which made it difficult for him (even in exile). They stole the girl he wanted (Caroline of Salerno, OTL duchesse d'Aumale), made it difficult for him to travel anywhere (July Monarchy told foreign courts not to receive him as "heir to the French throne" but only as the "comte de Chambord"), FORCED him to either remain unwed or marry the half-lame daughter of one of the few kings who thumbed their nose at Paris etc etc. Is it any wonder he hated them as he did?
@Emperor Constantine