Are there explanations for the morphological similarities and differences between languages? Which ones?
Yes, there is one, proposed here:
Donegan, Patricia J. & David Stampe (1983). Rhythm and the holistic organization of language structure. In J. F. Richardson et al. (eds.), Papers from the Parasession on the Interplay of Phonology, Morphology and Syntax, 337-353. Chicago: CLS.
This paper proposes that fundamental aspects of morphological systems and syntactic systems also are influenced by rhythm, specifically fore-stress versus end-stress. I don't have a personal opinion about whether it is a true explanation, since I don't know enough about the relevant areas. But it is an explanatory theory which has been fairly influential.
Here is an online version of the Donegan-Stampe paper: Rhythm and the holistic organization of language structure.