I think because a lot of mines are actually funded by folks looking for a way to buy BTC more cheaply than they can get it via exchange. Aminorex for example has mentioned in the wall observer thread how he has had contact with folks who contract with mines to buy their produced BTC for accumulation purposes.

Ok, so that works great as long as mines can produce BTC at a cost below the current exchange rate. But this essentially puts a floor beneath the exchange rate as the costs of mining continue to rise. Eventually the cost increases to a point where it might be actually cheaper to buy BTC on an exchange.

So from this view, the cost of mining and exchange price can be linked, with large BTC accumulators buying BTC from whichever source is cheapest at a given time.

As for hashrate being a proxy for cost, I think this is demonstrably true. In cases where hashing technology is roughly stable such in the CPU mining era, it quite clearly takes more silicon and electricity to mine a bitcoin if the difficulty factor increases. Economies of scale help, but the current large mining operations are already at that scale so there isn't much left to squeeze from that. If you search around you can find pretty good estimates of current mining costs, I think the current BTC price is in the ballpark for probably the overall hashrate, although newer mines are probably still slightly profitable here.