Very difficult since a lot of markets have supported minor pro hockey in one version or another for a long time and convincing them to pay the same price to watch teenagers play, regardless of talent level, is a massive undertaking. There's very few success stories for markets who made the switch, with Johnstown and the NA South Division being the only ones popping out. There's been many more who haven't taken to it like Flint (with both the O & NA), Bloomington, Youngstown is still struggling, Rio Grande, Fresno, and a host of others. Manchester NH is having trouble convincing people to see ECHL hockey after they supported their AHL predecessor, despite their sustained on-ice results.
Before the junior hockey scene on the East Coast got all jumbled up, the USHL supposedly was looking at several locations for an East Division, like Erie, since the McDavid-era Otters nearly relocated to Hamilton, and Johnstown along with Lewiston, ME and a couple of others.. They ultimately chose not to go that route for the same reason that 4/6 NAHL East Division teams rely entirely on income not derived from ticket and sponsorship sales. That reason being the northeast is too saturated with hockey, especially New England. People's ranked hockey interest is NHL/NCAA D1&AHL/ECHL/NCAA D3, and juniors would go well below D3. And the NAHL only made that move to counter the USPHL, something which the USHL wasn't willing to do or delegated to the NAHL.
There's just not enough markets like Lewiston, where it's too far away from the dozens of easy and cheap hockey options, who have an arena and tangible fan base to make the USHL go east. Lewiston hasn't even joined the NA because of their geographic limitations, the USHL would be even worse. The USHL is on an island and there's even fewer options going west and south than east. They keep travel cheap and relatively easy. Muskegon, Youngstown, and Fargo have some longer trips, but no one is regularly spending more than 24 hours in a bus like you see in other leagues. That's why most of the UShow teams are able to get by pretty nicely with 3,000 people per game. Expanding is highly unlikely and relocations are difficult since they can't be very far away from their cluster of teams or else the team is very quickly not financially feasible.