There are a number of reasons, but as pp have pointed out, most commonly people could have enough milk if they were given the right support. Not having the ability to produce enough milk is not common. Some of the major physical reasons why people can't breast feed include previous breast reduction surgery (might cause a problem but doesn't necessarily) and being on medications that are not safe during bfing. There is also a conditional where a woman can't stand to have a baby on her breast - this is an actual physical condition but is most common during pg. Otherwise there are various emotional reasons like previous sexual abuse. And of course the biggie, bad advice, lack of support etc.
And for the record, going back to work is not a reason to stop bfing. I continued to bf while working and know of others who have too. This might be harder for some than others due to the availibility of somewhere to express in the workplace, but in general it can be done quite easily - we just think it can't be done because there is not enough awareness of it.

