Because it's not just plain insulin. Insulin, for the miracle drug that it is, doesn't work for everyone equally. Depending on personal dietary habits, unique pharmacokinetic responses, how far diabetes has progressed... sometimes it lowers blood glucose too quickly or for too long or too short a time. So they've developed a hundred different formulations, and doctors basically just keep trying new ones until the patient gets the results they're both after. These new formulations cost more than plain insulin. They're almost all patentable and no generics are available. Wouldn't be shocked if old-style plain insulin hasn't (more or less) just followed inflation.