I somehow doubt that it was "push to buy the DLC". While it's a nice addition to them, "buy the expansion to see your pokemon follow you!" isn't exactly a good selling point.
It's most likely what
R_N said, without trying once more to throw smogonian reees at arbitrary nonexistant corporate greed.
The main Wild Area barely performs as it is (possibly a consequence of having had to rush the game out while still unstable), *expecially* with wifi turned on, adding more calculations for the following pokemon animations & route could have just not been worth the effort and they opted to stabylize it for the smaller "wild areas" of the two islands.
As for the routes, I'm confident that's just intentional, and the reasoning isn't hard to guess. Even without factoring the rendering distance issues, routes are both irregular, generally small, with plenty of stairs and jumps, which would have either required more complex animations or a constant appear/disappear so they opted to just not have them show. They don't show in any of the non wild-area sections of the two DLCs either.
The constant appear/disappear effect is something you can easily notice on Let's Go (where you can have the following Pokemon everywhere) and honestly it's really obnoxious in some parts, I can definitely see a good reason to not include it.