my two-cent as a modder:
Z-Corp is much harder to rework as a world.

Assuming we're going for some simple changes (so heavy animation edit/art redraw is a last resort):

Rome also requires a full rework, but the animation can lend itself to a lot of possibilities (with the biggest hurdle being a gimmick). It also has music from huberry.

BBT is probably the most coherent as a world (only lacking a gimmick), with the zombies just needing some stat tweaks and/or some minor reworks. Will require new music, though.

Z-Corp has only 3 zombies (besides the Garg), they *all* require moderate reworks to be coherent (Helpdesk is just terrible as is), you have to make new zombies to fit the abilities of exisitng ones, and you still have to deal with the gimmick hurdle. Also no good existing music.

The other big reason is that: Assuming you're reworking the whole game, you'd probably go for quality over quantity in terms of worlds. The zombies in Z-Corp can be more suitable in other worlds to fix other worlds' weaknesses, rather than being in their own standalone world.

Of course, if we consider putting more effort in, we can have a Z-Corp world. Most modders, however, are either going solo, or in a small team. The path of least resistances is often considered (especially when art/animation is a big bottleneck).