There is no particular reason for it, from what I can tell here are the similarities.
Most are from Gen III: Driftblim/floon are the exceptions
4 families have high HP: Wailord, Hairyama, Swalot, and Driftblim
Aside from Seviper and Illumise they all are part of 2 part evolutionary lines.
None are legends as you said.
Most have uncommon base stat totals.
But it seems that they were random Pokemon were dumped into it. I will look into it a little more.
EDIT: I have found a potential reason, many of these Pokemon are not particularly hard to get at low levels in the game (correct me if I am wrong), so they can level up fast to lower levels to serve an ingame purpose, also most evolve in the quick leveling up part(or near it) of their fluctuation. Legends on the other hand are strong and are found at high levels.