Imagine you had a large application, and wanted to add this library to it. To comply with the GPL, your application would have to be under a GPL compatible license. Such applications are a smaller subset of all applications hence the question.

The LGPL does allow for the library portion to be GPL while the rest of the app doesn't have to be, but that isn't the license used in this case. Typically the LGPL favours more widespread adoption, while the GPL favours freedom at the cost of some adoption.

LLVM also has a JIT and a far more permissive license, making it more attractive to those not using a GPL compatible license.