JBPBRC wrote...
But why humans?
The answer is subtextual.
The codex entries on the Reapers indicate they're particularly picky about which species, and specifically what members
of each species, are used for processing into Reaper capital ships. Given the exposition, it's based upon genetics though there may be other factors. Species and individuals who aren't suitable, regardless of reason, are husked, used as indoctrinated sleeper agents, a menial labor force, or processed into destroyers.
Humans, for their large population and wide genetic variance (as exposited in ME2), are a strong candidate for being processed into a capital ship. Given they're "next door" to the batarians and for their comparative lack of military, industrial, economic, and political strength compared to the other Citadel races and therefore easier to pacify, it's only logical to eat the humans first, make the capital ship(s) and proceed with the rest of the extinction.