Given the geography (Alps!) and available roman roads, there were only a few ways to cross into Italy:
1) Via Cottia per Alpem, coming in from NW near Susa, Italy (312 AD route)
2) Via Franigena, coming in from NW near Aosta, Italy
3) Via Iulia per Alpem, coming in from north near Chiavenna, Italy
For either of the first two, it would seem a bit perverse NOT to have visited nearby Lyons.
As-the-crow-flies the nothern 3) Via Iulia per Alpem would have been more direct from Trier, but I don't know what roman roads existed in Gaul. There must have been a reason he chose to come in via Susa in 312 AD. Anyways, he was coming to celebrate, not in any rush. He spent at least 3 months in Rome, since Depeyrot has him still there in late September. So, one might ask why would he not have chosen to visit Lyons on the way. Presumably roads were good between Trier and Lyons, both being major cities.