It's likely to be about how to position everything within the AM4 platform module:
Ryzen 2500G:
Ryzen 3900X:
Space would be tight on the Zen 2 layout to have big Vega chip in there, but drop a chiplet and there could be room for a little Navi chip. Given that it's only just launched, it makes sense that AMD will be waiting a while before refreshing their APU line.