Porcupine distribution is interesting. The species seems to have declined drastically in some parts of New Mexico (although its range is statewide) and a working hypothesis is that local extirpations are due to increases in Mountain Lion numbers in those areas as lions will prey on them. I’ve seen some possible evidence of that in one location where lion numbers went up greatly since the 1990s and the porcupines I used to see there are now gone.
My boss is in Midlothian and had one in his yard last winter; I still haven’t seen or any sign in Dallas proper or out in Richardson (my area) but man oh man I’d love to see more of those guys. I miss them so bad from the panhandle
in Texas, folks say that west of I-35 is dry and east is humid. that roughly corresponds with the arid / humid climate boundary that historically has been at roughly the 100th meridian west. it’s been noted that dry has been shifting eastward in recent years, perhaps related to climate change, and that, in turn, could be related to porcupines moving eastward, too. dry / humid probably isn’t the whole answer, since it wouldn’t explain porcupine distribution at more northern latitudes, but that’s where i would start if i were looking for explanations.