It is all because the seed coat becomes tougher and more impermeable to water.
Baking soda (1 teaspoon/qt) can reduce the cooking time by nearly 75% as it contains sodium and in addition to being alkaline. Alkalinity facilitates the dissolving of the cell-wall hemicelluloses as in producing hominy from dry corn. In the case of “hard” beans, both salt and baking soda will speed cooking.
The main drawback of baking soda is that it affects the taste, texture and can also result in destruction of a lot of nutrients. A better was would be to just pressure cook them although that still takes time. The quickest way is to grind the beans and cook them.
Grind pinto beans until fine using a food grinder. Place in a saucepan, and whisk in salt, cumin, chili powder, and warm water. Bring to a boil over medium heat, and cook until thick, about 15 minutes. When the mixture has thickened, stir and cook for 4 more minutes. Remove from heat and stir in salsa. Serve as a dip, or use as filling for burritos