Maybe these are of some use:
Tauli, Valter "Standard Estonian Grammar" I, II. Uppsala University 1973, 1983.
Harms, Robert T. "Estonian Grammar" - Uralic and Altaic Series, vol. 12. 1962.
Erelt, Mati (ed.) "Estonian Language" – Linguistica Uralica Supplementary Series, vol. 1. 2003.
But as for why
grammar is as it is, I am afraid the most common answer would be something like "because for reasons unknown to us it has historically formed to turn out this way"