I've been playing around with this a little in my head, and I actually think BOS Logan landing and gate fees may be a little too low to incentivise the ULCC carriers to go to ORH and the relievers instead of Logan. If you compare the service footprint of the ULCCs (Frontier, Allegiant, Spirit, Sun Country, even SW) at the secondary/tertiary airports of major cities compared to the primary, they tend to dominate. BWI instead of DCA/IAD, Stewart and Long Island Macarthur instead of JFK/LGA/EWR, Trenton instead of PHL, MDW/Rockford instead of ORD, San Jose/Oakland vs SFO, etc.
The Boston area seems be an exception, where the cheaper distant secondary reliever airports don't see more ULCC service than Logan. PVD, MAN, ORH, etc. Perhaps higher average fares make it worthwhile, idk.
This isn't necessarily a bad thing for Boston visitors / residents, or the Boston economy, mind you - it's just curious that Allegiant for example found it better to pay Logan rates vs continue at worcester, and that the carriers that have gone are the majors and not the ULCCs, given that the worcester runway is, as I recall long enough for a 737/a320 type. Logan doesn't exactly have (in a pre pandemic world) a particular excess of capacity either. That JetBlue has made non feeder Ft. Lauderdale and Orlando work, speaks to the fact that Allegiant and the ULCCs could have too.