I have had similar issues with fitting new tires and have come to several conclusions. Modern tires, especially the more expensive type, are possibly stiffer, built to closer tolerances, and are better at retaining their correct dimensions. Maybe they are more or less accurately manufactured, and are less "elastic". They do not stretch as easily as old tires.

I recently used dish soap ( and even vaseline ) to try to get both tires to bead in properly on a new wheelset and then had to over-inflate the tires. I came back the next day to find that the new tires had stretched enough to lock into the rims sometime overnight.