Similarly, according to Henri Cohen (personal communication), the leading number theory program GP/PARI was licensed under the GPL because of the dependency on

Similarly, according to Henri Cohen (personal communication), the leading number theory program GP/PARI was licensed under the GPL because of the dependency on Readline. RMS personally requested that Cohen change the license of GP/PARI to the GPL when visiting France long ago. I licensed SageMath under the GPL initially, since it heavily depended on GP/PARI, so I didn't have a choice. That said, the GPL makes a lot of sense for software such as Pari and SageMath.