IMHO the absence of wing guns was to better the rolling inertia (decreased). The motor-canon was not ready at early stage of the war. Hence the assisted starter trough spinner was mounted instead.
@Kongo-Otto who said " Realy? And your sources are?" speaking about 109 with wing's canons belt fed mechanism : you can buy a copy of "Le fana de l'aviation" issued 3 or 4 month ago.
Have a good read