: It looks like they're reusing old Bungie-era assets for Grunts, Jackals, and
: various weapons in the H2A campaign, and even the ones that aren't direct
: copies look like reasonably faithful to Bungie-era Halo.

Apparently they want to keep it consistent with H1:Ani hence the very similar visual artstyle. H2 also has a lot of unique assets that weren't in later games, so it's not really surprising they are doing this as it all needs to fit the Bungie aesthetic.

Also a lot of H4 assets don't work well with the older rigs and proportions, for example getting the H4 chief rigged to the H2 boneset would be difficult as there are some differences in the size of parts of his body. And then you have animation clipping to worry about too.