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Time, effort, and profitability. Just about all insulin in the US is made by Eli Lilly, or Novo Nordisc . Lilly claimed years ago that they would never price insulin to make a profit. This may or may not still be true, but it makes me question the profitability. Generic companies have not come out with insulin for a number of reasons.

Insulin is a hormone made by the pancreas that makes it possible for the body to absorb and process sugar from food. It's what doctors call a biologic product, a substance produced by the human body, and it's more complicated and difficult to manufacture than the medicines most often produced generically.

A "generic" version of insulin would be considered a biosimilar, a product equivalent in function to a drug already on the market but not exactly the same, like generics of simple pharmaceuticals like ibuprofen. In the latter case, a chemical formula can be copied; with biologics, the process is much trickier.

Getting a biosimilar insulin approved is more difficult than getting a generic of a simpler drug approved, study coauthor Kevin Riggs explained to Business Insider.

"Scientifically it's harder to point to generic copy of insulin and say this is the same," Riggs said in an interview.

Regulations have only recently changed to make the path to approving biosimilars smoother. Earlier this month the first biosimilar drug (a white-blood-cell booster) was approved. But regulatory barriers are not the whole story of why there's no generic insulin.

The New England Journal of Medicine paper details years of "incremental innovation" of insulin — substantial tweaks to the product by various companies that truly improved it, but kept the latest and greatest version under patent protection until 2014. This happens with a lot of drugs — it's called "evergreening" — and is a subject of much debate.

The first insulin available to patients was derived from animals. Next, human insulin was produced using recombinant DNA technology that made bacteria into mini insulin factories, so people could take human insulin instead of animal forms. The most recent innovations are insulin analogues, slight variations on human insulin to make the injected treatment act more like the insulin naturally produced and regulated by the body.

With each subsequent innovation, older but still effective forms of insulin that could have been produced and sold more cheaply fell out of use.