Being split by the Carpathians.
Give them fortification technology and have them fortify the Carpathians and they are extremely defended.
Think it had to be more than that or there would have consistently been only a couple of polities instead of being more tribal federations or a plethora of small kingdoms, with an occasional high king---they seem to have only united twice under one king.
Fortification tech? They seemed to have it. The land was full of mountain forts and their capital, Sarmisegetusa, had sophisticated fortifications that required a siege, the full panoply of Roman siege technology and 3 legions to overcome.