While I'm no EB developer, I know that to include all those units (if I remember correctly RTR has hastati, principes, and triarii for Samnium, Picenum, and Etruria) would take up 9 unit slots that could be used elsewhere. Additionally, that degree of diversity is out of place for such a small area (e.g., the Italian peninsula).

Also, think about the costs and benefits. By adding diversity to the early roman republican armies, you lose diversity somewhere else. And, selfishly, because I usually play as Pontos, I like a lot of diversity in the Seleukid, Ptolemaioi, Makedonian, Greek, Epeirot, Getai, Sauromatae, and Hayasdan armies.