SASLess - the main reason the CH-53 and the 101 haven't made it is cost & complexity. The -53 is a great helicopter but it hasn't even sold to many airforces.
There are issues about having to upgrade the helidecks on rigs etc and you'd need a much longer contract than is usual to make economic sense. Range rather carrying capacity would be the main attraction - if people get way out (say the southern end of S Falklands Basin) then they would be attractive - but if you are only drilling 3-4 wells.................