Because most small caliber ammo is made in production runs. Something like 30-06 might get ran for 1 day a year. Now add up all those different odd calibers and think of the time to switch out dies in presses, feeds for power, feeds for the projectile and you're talking about real numbers of lost productivity where they could be cranking out .223 and .308 the entire time instead.

Also, other shooters will shoot the common rounds like .223, when it's cost goes up or it's unavailable, they move to these other rounds increasing their costs.