Recently, I learned that pandas have a carnivore's digestive system but that it exists almost exclusively as a herbivore. If pandas hunted would they be able to find meat in their environment? Can't think of any reason why they would 'evolve' to eat only bamboo if this was true. It's like 2 million years ago some bears decided they wouldn't hunt, hibernate or even roar and are slowly committing suicide.
First: My opinion on why the question has received that many down votes
There were many down votes to this question, not because the question is no of interest. The question actually makes sense and is interesting. The post attracted those down votes because the post and the comments of the OP shows little understanding of evolutionary processes. Here are some issues:
some bears decided. Evolution is not directed. There is no conscious decision from any species (except humans, see eugenics) on how to evolve.
as far as I know humans are the only animals that don't live instinctively. I am not sure there is a common definition of the word "instinctive". But I very much think that the vast majority of people would suggest definitions that forces us to accept that humans behave instinctively. And if the definition would accept that human can sometimes not behave instinctively, then the definition will very likely accept that some animals behave instinctively.
why they would 'evolve' to eat only bamboo. Why did you use quotes?
Eventually the question may show lack of effort as showed in the fact that the OP is stating the 2MYA the panda and the other bears diverged, while it was rather about 36 MYA (according to onezoom and even a bit older according to this reference). This paper however shows that pandas had an omnivorous acenstor 7 MYA only suggesting that pandas modern day diet has evolved less than 7 MYA. More information below.
I don't think that those evident misunderstandings deserves receiving down votes (not sure though).
Can't think of any reason why they would 'evolve' to eat only bamboo
I had a quick look and I didn't find any good answer to your question. It seems that:
Pandas have a digestive track that is very short (like most carnivorans and unlike most herbivorous). Because of their short digestive track they eat a lot (between 9kg and 14kg of bamboo per day!) and they poop a lot (40 times a day) wiki!
As a consequence of its diet, the panda spend little energy and avoid any effort wiki.
Pandas seem to have evolved so that it is able to digest cellulose with the help of bacteria (ref, ref). According to wiki and BBC this microbiome is not herited from the mother but is acquired later in life (it is quite common to see that). This paper suggests that having such a fast transit prevent the ability to absorb other nutrients and therefore limits the benefit of having a more diverse source of food.
pandas size and mouth are adapted to their diet. They have a large size and a round face with powerful jaw wiki.
According to many comments online and according to wikipedia (but I couldn't find a trustful reference), as observed in zoo, pandas can eat other stuff when available. Citing from wikipedia
While primarily herbivorous, the giant panda still retains decidedly ursine teeth, and will eat meat, fish, and eggs when available.
Given the amount of information we're having there may be tons of possible scenarios. It is possible for example that there is inter-species competition in the wild for access to prey. It is possible that there was for some point a very low density of preys and pandas has shifted their diets. Once, pandas had evolved some features of herbivorous species, they would get stuck there and not be able to get back to a meant based diet (stuck onto a local optima on the fitness landscape). There might be many different explanations involving patterns of competition, parasites, sexual selection, etc... And it seems that we haven't been able to disentangle between all of these.
Hope that helps!
At approximately the same time that they switched to a primarily herbivorous diet, pandas lost the ability to taste "umami," or "savoriness." We don't know whether this happened before or after the dietary change, but the umami flavor is a strong attractant for other meat-eating omnivores, like humans. It signals instinctively that a food is exceptionally valuable and should sought out and prioritized over other available food. This is why your mouth waters more at the smell of a steak than the smell of a carrot. Unable to distinguish this quality by smell/taste, pandas would be less inclined to go to the effort to hunt down or scavenge meat, compared to just stuffing readily available but almost inedible bamboo into their craws for sixteen hours a day, even though it often gives them serious indigestion and limits their ability to travel or reproduce.