The individual Sea King can carry more troops, but the troop-capable Sea King force in PN is very small.
The alternate approach is to have more helicopters available -- albeit with a medium-lift (12-16 troop) -- capacity on hand. The operating cost of the AW149 is also lower thanks to heavier COTS use. In the long-term, it's easier to add more of this type than specialized platforms like AW101 and AW159. Granted, the ASW/AShW variant of AW149 isn't going to carry troops; but the two variants would still be identical from a maintenance and support standpoint.
Besides, the risk of the AW101 and AW159 is actually long-term operating cost, it will be higher than the AW149 (the latter was designed to be low-cost upfront and through-life). Finally, having more helicopters on hand helps offset for downtime and non-availability rates. The PN is (or at least was) thinking along the Sea Hawk and Black Hawk lines.