Been reading with interest. My own perception is based on what I've seen at SOU- I believe some routes can sustain frequencies greater than those currently on offer.
Using Eastern as an example, their SOU-BHD route has been steadily building and now has strong loads at healthy fares and full or almost-full aircraft on the peak travelling days.
And yet, the date when it goes double-daily keeps getting pushed back by a month at a time.
Given that it's the same couple of crews flying most days, my impression is that it's a combination of lack of crews as Ozzy alluded to, with the current health and economic environment contributing but not solely responsible for the slow ramp-up of flights.
SOU-MAN has lower loads than SOU-BHD, but given the frequency and cost of flights I don't find this surprising, and even then they're probably averaging about 40-50% full, which seems to be a standard Eastern load. I've got no doubt that with cheaper fares and higher frequencies we'd be seeing much busier flights.
SOU-DUB loads are building steadily too.