ACN having two Jobs failed because of an oversight on SE's part connected to the change to ARR - in that each class/Job's abilities were specifically controlled or guided by one stat, thus players would naturally pump most of their AP into that stat (like, MND for CNJ/WHM). This worked fine for a single Job, as each Job generally refined the role of it's based class and thus continued to make use of it's controlling stat.
ACN though, which was officially designated as a magic DPS, (and thus INT would be it's guiding stat), suddenly had a healer Job option in addition to it's DPS Job - thus those who put all their AP into INT and wanted to play as SCH found themselves stuck, as their healing spells (which required high MND) was neutered.
A band aid solution was found by reducing greatly the price for Keeper's Hymns (which allow the player to reset a class's spent AP), but it wasn't really fixing the root cause of the issue. Either way, it showed too many flaws with the system and thus SE abandoned trying to give other classes multiple Jobs, leaving ACN's two Jobs as an orphan, an experiment that just didn't work as well as they had hoped.
As for the Heavensward Jobs having no base class and starting at level 30, that was born from an entirelly different cause and was yet another failed experiment on SE's part - as the new Jobs were gained from Ishgard that not only required level 50 access, it also did not have a level 1 area next to it - the min level for enemies in Coerthas Central Highlands is around the mid 30s. And given the fact the expansion's storyline kind of has the player on the run from the starting cities, it created a tight spot in providing a location to level new classes and Jobs with. Thus, the new HW Jobs were made as separate 'Extra Jobs' and not regular 'Disciples of War or Magic'.
Again this hasn't worked as well as SE were hoping - one of the main reasons for even having levels is it allows the player to gradually learn how to play as the class, starting with one or two basic abilities and then as they gain exp learn new abilities that compliment what they've already learned. With the HW Jobs however, that isn't the case. Players are suddenly chucked in the deep end and given 30 levels worth of exp instantly and told "Off you go, it's up to you now." Although most players did get the hang of the new Jobs more or less eventually, it was still a rather unfortunate affair all round and showed just how poor this idea really is.
Really (and I've mentioned this in a few earlier threads), in a perfect world I would love SE to actually bring consistency back to the Armoury System, and change these experiments to bring them in line with the rest of classes/Jobs - starting with removing SCH from ACN and giving it's own new base class (Mediator?), and then add three base classes to the Heavensward Jobs, thus changing them to proper Disciples of War and Magic - starting with bringing in the long dummied out class Musketeer which would be an obvious choice for MCH, with AST and DRK getting, say, Mystic and Ravager respectively? With that done, I would love to see maybe a change to the base classes made by removing cross-class limitations on class abilities while a Job crystal is not equipped, thus truly allowing players to experiment with customization, and leaving Jobs for duties, dungeons and raiding. But, such an idea is probably sadly unlikely by this stage (*sighs sadly*).