Tony, I was wondering the same thing – why not adopt that sort of heavy cavalry much earlier?
The greeks and presumably the macedonians definitely knew about barding long before Antiochus' eastward romp – according to Armies of the Macedonian and Punic Wars, Xenophon described Persian cavalry using barding in 401 BC… close to 200 years prior to Raphia. Xenophon apparently even recommended adopting persian cavalry equipment for greek forces, but was ignored.
The greeks in Xenophon's time not adopting barding is easy to understand, as most of greece wasn't great cavalry country and infantry was their main strength.
For the successors, it's a bit more of a mystery, as Alexander definitely considered heavy cav the decisive arm and the later kingdoms probably did too. I wonder if the main reason the successors didn't adopt barding and cataphract-style equipment might be that barding was developed as a counter to archery, not as a force multiplier for shock cavalry. Basically, the horse is the main vulnerable part while the lancers close with enemy infantry or mounted archers, so layer some armour on the horses… basically the same scenario that drove the adoption of heavy barding by the French during the 100 Years War.
So, Alexander's Companion heavy lancers and their descendants were more than adequate for wars against forces that were mainly heavy infantry (greeks and the successor kingdoms) – and thus no barding. But when Antiochus went east into areas where mounted (and dismounted) archers were a serious threat he saw the value in adopting barding – the local anti-arrow technology – and added it to his agema.
Once the focus shifted west again, vs the romans, the usefulness of barded heavy cavalry seems to have dropped again. The romans were never shy about adopting enemy kit that they found useful, but they mostly ignored barding for a couple hundred years – until they were faced with an expansionist threat from eastern horse archers (steppe nomads and later the Sassanids).
That's all conjecture on my part, but the scenario could fit.
From a wargaming standpoint, we might be making a mistake in ancients rules to assign barded cavalry additional advantages in close combat if barding was actually a defense against arrows…