Because market inefficiencies make certain necessary adaptations effectively impossible.
For example, if Company A decides they want to be responsible corporate "citizens" and shift their energy consumption to sustainable sources, then they increase their costs and can no longer compete effectively with Company B unless there's a mass movement to purchase A's products because of their energy policy. And unfortunately the existence of Walmart and the like is proof enough that the mass of Americans consider up-front price to be the single most important factor in purchasing decisions, even when it increases their own long-term costs (a $50 appliance that needs to be replaced yearly is far more expensive than a $200 appliance that will last indefinitely), much less indirect social costs whose full weight won't be felt for generations.
Granted, at the moment if we removed all fossil-fuel subsidies renewable energy would look far more competitive, but to really level the playing field we would have to also impose new penalties on "socialized-cost subsidies" that have long been grandfathered in: Coal for example imposes phenomenal pollution costs at almost every stage. If however we imposed well-structured penalties/taxes to reflect the actual cost of reversing that damage then it would be one of the most expensive energy sources available.