I believe I may know the reason for the problem. But I could be incorrect.
CX does not do its own upgrade or maintenance. Instead, it outsources to HAECO, which both CX and HAECO are Swire's subsidiaries. Because of this relationship, CX is not exactly a paying customer. On the other hand, HAECO does have some real paying customers. On that basis, HAECO may place a priority on those customers instead of CX.
Also - Wifi does not yield a large amount of profits. So it does not make sense for CX economically to ground a plane just for the WiFi upgrade.