I think the main thing is that the herbivores are highly specalized to a locality or type of terrain, while being further limited to an area by where the nukes and radiation did or did not allow for survival.
If we were to have a full map of the lower 48 I imagiane we would find many localized critters across the foodchain, some are specalized to a terrain type and wouldnt expand past it, so no mudcrabs on the west coast, but we see them in F3/F4.
Deathclaws were evolved from a type of lizard, but the enclave also spread them around so you see them pop up elsewhere, plus they're apex as fuck so readily establish populations, aided further by the fact that they are rather smart, given that a little tinkering can grant them sentience.
I would imagiane that if the south were to be featured in a game, from texas eastward, you would see feral RadHogs as a persistent threat and source of food, with subtypes existing in the more irridated areas that are less food and more radioactive.
Indeed, RadHogs are something that could be argued as something that could reasonably start showing up out west post new vegas.
Farm animals like chickens would be easy enough prey and succumb to radiation easily enough so that they would likely have mostly survived in places that maintained a captive breeding stock, with any released into the wild post war needing specefic terrain to have a chance to survive.
Actually I think a great deal of the critters that were domesticated or farm stock pre war would repopulate in clusters based around areas that maintained a stock of the creature in safety, but that would make them localized to that area.
The key thing to remember is that the later you get in the timeline, the more chance things have had to recover, plants are growing back, smaller critters largely lacked the body mass to survive the radiation so even roaches and flies mutated larger over time, and as human populations exited shelters they would bring their domesticated food animals with them to the surface, and those creatures would start being reintroduced, but the less mutated creatures would not likely survive as readily.
Id say that scorpions lizards and vermin would be the source of some of the most common creatures.
Radscorpions spread coast to coast, which would imply they were naturally some of the more hardy in the conditions early on.
Rats of various types, molerats etc, also did quite well, and could be domesticated for food.
Geckos would also be common as a threat and as food source, and their appearing on the east coast should only be a matter of time, same for the giant mantis.
You also have to consider the sheer scale of the continental united states, there would be lots of places like zion valley, largely unaffecred directly and quick to recover, where the major costal cities would have been heavily nuked, and we know from Tychos dialouge that there was civilization anew in parts of the midwest prior to Fallout.
Bassically, the world is recovering, albiet greatly changed.
Looked at from a meta prespective of the local wildlife:
Fallout: we see a chunk of the west coast.
Fallout 2: We see a bigger chung of the west coast, with overlap giving us a picture of how the northern area of Fallout has changed.
New Vegas: We see a desert region of the west that mostly avoided nukes, and has recovered fairly well given the sparser wildlife to be expected in such terrain and in DLC we also see Zion cannon, an isolated area that bounced back quite well too.
Fallout 3: We see the east coast and probably one of the more heavily nuked areas in the US recovering much slower.
Fallout 4: We see another area of the east coast that wasnt nuked as heavily, far more recovered, we also get to see a coastal island thats recovering nicely.
So, based on this pattern Id say that we should start seeing areas with plantlife and tree coverage, plants bouncing back mean smaller herbivores and omnivores have more food sources and more places to hide.
