big block vs. smal block

The trans costs the same. The differential costs the same. Big blocks are cheap. I've gotten complete 440s for free on Craigslist. A V8 K-frame with discs are $200 to $250--same price if running a small block. B-body HP manifolds for $150. 340 manifolds go for $500 to $800 plus $130 for the drivers side gasket alone. Why were big blocks made in the first place?