Hi,
I would not compare so easily a 2499 to a 5970 -produced at different times in history of watchmaking and watch market- but I can remind that the 2499 calibre was based on a Valjoux calibre, like the 5970 calibre is based on a Lemania calibre!
Rarity is a matter of offer and demand. Offer (in volume) was much lesser due to the way watches and watch calibres were made. Demand was probably lower too for the 2499 in volume, but maybe not so much lower relatively to availability of the product.
The 2499 demands more because it is a vintage watch which production ended long ago, and long before the booming watch we've known more recently! 
Just my POV of course!
Cheers
Dje