Because negative pion is not known hero of strong and weak interaction.
Brefly - in 1932-1934 James Chadwick which knew no negative particle except for electron, told to physical community that neutron cannot be composite particle consisting of proton and electron. From this obvios fact he made speculative statment that in this case neutron is elementary particle like proton. Though it is not logical, physical community believes it very much after almost 100 years.
That makes impossible to understang real role of negative pion in weak and strong interaction. As well that belief makes impossible to understand that proton and negative pion compose atomic nuclei. And neutron is just composite of proton and negative pion.
As well it makes impossible to understand that negative pion inside neutron and atomic nuclei is responsible for beta-decay.
You look at most important particle from the point of view of nucleus when you ask about negative pion beta-decay. Although electronic (not muonic) pathway is most important. As negative pion inside nucleus cannot decay to muon - binding energy of negative pion to protons is bigger that energy of muonic pathway. Electonic pathway gave energy enoph to be favorable for neutron and some nuclei.