As you noted in FFXIV it's always been called Musketeer, and was a class that had been meant to be included in version 1.0 as well (like ACN was), but was dummied out (although it's class symbol still appeared on the map).

I can only speculate the reason for MSK's long history of 'no shows' in FFXIV is simply that both development teams (firstly the original 1.0 team under Tanaka and Yoshi's team that took over) just couldn't make the class 'unique enough' from simply being a gun-using copy of ARC - both ARC and MSK are ranged Disciples of War, the only real difference is the choice of weapon, one uses bows and the other guns.

It should be noted this was back in 1.0 days when Jobs didn't exist, you could cross class any ability on any class usually with little penalty, and ARC actually had to equip consumable ammunition, which due to the predicted high cost of having to craft probably was another nail in MSK's coffin.

Of course with the change over to ARR, those issues probably are no longer relevant, so it will be interesting to see how MSK makes it's debut (and it will, even ignoring the thinly veiled references at the Fanfest - the Coral Tower still has the MSK class crest inside it in ARR.)