Do they even want it? People is divided on it and next administration may go all nope on THAAD. What then? Will US take it down and out of SK? What if even current administration says no to bill?
Honestly though if I was in charge of that decision and it was basically free or overly cheap then why wouldn't I?
a) I get a better defense system while other countries wished they had it or payed billions for it.
b) US forces are already stationed in SK and this deal seems not to be pushing for more of a US presence. (rather it is the US interest to keep SK safe)
c) Let us not kid ourselves and believe that if the US didn't install this then NK would be more chummy and less likely to develop their missile/nuclear capabilities.
d) Regardless of having this system or not NK can nuke SK but does not do so due to not having the ability to nuke the US, who would completely nuke NK or invade them if NK tried anything.
NK saw what has happened to other countries with dictators and basically already knows that their self existence is on the reliance of having the ability to strike the US and cause MAD. (for at least some US cities). They will continue the path to more nuclear capabilities since that is the only logical approach for them to continue their regime.
Gaddafi dropped his program due to fear of an invasion and by pressure as well, only to be invaded via a Nato airforce later on. Saddam was invaded for the gist of "chemical weapons". NK is isolated even more so then before, the USSR doesn't exist anymore to maintain MAD for them and China sure as hell isn't going to sacrifice itself for NK.
So individuals basically saying no, either want US forces out of SK and are hopeful of NK basically backpedaling and somehow becoming chummy and rejoining with the South or they have no logical reason to refuse what is basically an advantage for not only NK but also other potential missile attacks to SK in the near future.