Because the cost of electricity required to mine competitively is variable. When the price of bitcoins goes up, mining becomes more profitable. More hashpower i

Because the cost of electricity required to mine competitively is variable. When the price of bitcoins goes up, mining becomes more profitable. More hashpower is added by new and existing miners until it no longer makes financial sense to do so. The value of a bitcoin is almost always near the cost of mining one, but it's the value that determines how much mining there is.