Why do Giant Pandas eat essentially only bamboo when it offers them so little?

Bamboo may have low nutritional value, but it is very abundant in the Giant Panda's habitat.  Of course, if the habitat is destroyed by deforestation, the Giant Panda will have nothing to eat.  The Giant Panda have specialized on this plant because of its super abundance.  Despite its low nutritional value, it is available year round and the animals do not have to look for it.  The only difficulty comes when bamaboos flower once every several decades and then die en mass.  During this critical period, the Giant Pandas need to be able to migrate to other bamboo forests to find food.  The increased fragmentation of the Giant Panda habitat makes this more difficult.  The Chinese Government, however, has recently provided corridors that connect different patches of bamboo forests so that the Giant Pandas have a better chance of surviving famines caused by flowering of the bamboo.

Think about it this way.  Bamboo has low nutritional value, but it is very easy to find, almost constantly available and the panda has pretty much no competition for it.  If it was to eat something of more nutritional value, it would have to spend a lot more energy travelling and finding the food and maybe fighting off competitors.   For instance, nuts and berries are nutritious but they are only produced by trees at certain times of the year.  Grubs and ants are nutritious but you need to eat a lot of them if you are a bear.  Even if it did eat nuts, berries, grubs and ants as other bears do (eg. Black Bear), there would still be the ecological niche of eating bamboo available, so another creature would adapt to eating bamboo.  

Also, animals don't hibernate because it is positively beneficial, but because there is not enough food available at certain times of the year in northerly latitudes (the same reason that we sleep - we cannot get food at night, so it better use of energy to sleep).  Pandas don't need to hibernate as their food source is available all the time and it doesn't get too cold.  So really, like any specialised occupation, bamboo doesn't offer them so little.  It is a reasonable deal.  Hope that helps.