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process will only take up about a minute of two of your time.
I'd imagine it's beucase the vocative is the same as the nominative in all but the second declension, it'd be a bit strange to cram the voc at the very beginning when very few people are going to check to see what it is. As for gen before acc, most reference dictionaries I'd say all but I'm sure there are counterexamples give the nominative form, then the genitive form afterwards, e.g. rex, regis or rex (-egis) or some variation thereof. Since reference dictionaries give the nom and gen forms linked like that, it'd be sensible to place them beside one another on the declension table. For students, it'd help make that link so they could check dictionaries later on.