well, i think there are a number of reasons why united may choose to fly only weekly to some of these destinations:

1. the traffic on these routes may be heavily biased towards the honeymoon and cruise crowd
2. if they fly the route 7x/week, they'll have to dedicate a plane to the route. if they fly only on the weekend, they can pull a plane from a "business" route (i.e., route that performs poorly on the weekend). that way, they can serve two routes with a single plane
3. since some of these destinations are new, it may take time to establish tour packages with the local hotels, etc.
4. united may be focused on preventing rivals from gaining potentially lucrative route authority -- the idea being "get route authority now, figure out schedules later." see, e.g., the recent squabble among united, allegiant and champion over the denver - cancun route.