The best answer I've seen to this question comes from a book that is almost 40 years old. The Logic of Collective Action by Mancur Olson explains why some groups are able to have a larger influence on government policy than others. I'll give a brief outline of The Logic of Collective Action and show how we can use the results of the book to explain economic policy decisions. Any page references come from the 1971 edition of The Logic of Collective Action. I'd recommend that edition for anyone who is interest in reading the book as it has a very useful appendix not found in the 1965 edition.
You would expect that if a group of people have a common interest that they'll naturally get together and fight for the common goal. Olson states, however, that this is generally not the case:
"But it is not in fact true that the idea that groups will act in their self-interest follows logically from the premise of rational and self-interested behavior. It does not follow, because all of the individuals in a group would gain if they achieved their group objective, that they would act to achieve that objective, even if they were all rational and self-interested. Indeed unless the number of individuals in a group is quite small, or unless there is coercion or some other special device to make individuals act in their common interest, rational, self-interested individuals will not act to achieve their common or group interests."(pg. 2)
"Since a uniform price must prevail in such a market, a firm cannot expect a higher price for itself unless all of the other firms in the industry have this higher price. But a firm in a competitive market also has an interest in selling as much as it can, until the cost of producing another unit exceeds the price of that unit. In this there is no common interest; each firm's interest is directly opposed to that of every other firm, for the more the firms sell, the lower the price and income for any given firm. In short, while all firms have a common interest in a higher price, they have antagonistic interests where output is concerned."(pg. 9)
Be sure to continue to page 2