I am sure that most FOSS contributors do it out of a motivation other than license restriction, but I am also certain that many potential FOSS contributors work for organizations that may have questions about contributing to a GPL project (not particularly valid concerns/questions, but the less you have to deal with the legal department the easier it is to get sign-off on contributing to a project...) When you are talking about semi-specialized software like numerical analysis tools it is quite possible that there is a latent pool of potential contributors in industry who would find it easier to contribute to a BSD/MIT project than a GPL one.
It is also the case that this is basically a library, and many who would have no problems using/contributing on a GPL application will balk when it comes to a library or framework.
It is also the case that this is basically a library, and many who would have no problems using/contributing on a GPL application will balk when it comes to a library or framework.