Dune is a relatively new synth, so we made it Intel-only from the start. Older synths may still be UBs.
There is too many Mac versions to maintain already (AU-32, VST-32, AU-64, VST-64, perhaps later RTAS/AAX), and besides we don't have any machines with a PPC CPU anymore. Furthermore I'd expect it to be rather hard to get acceptable performance for DUNE on the PPC without heavily tweaking/customizing the code, which might take a lot of time. For our older synths this is less of an issue, as they're more simply structured and consume less CPU power.