What's wrong with job/subjob?

Quite easy. For one, it's excessively limiting and the number of viable combinations is not that high. An inevitable consequence of that is that cookie-cutter be all end all combinations tend to be the only ones accepted (with reason), making the freedom of choice between job/subjob a completely illusory one, and as such moot.

On the other hand the FFXIV armory system offers a much higher amount of combinations, and while a certain amount of cookie-cuttering will always exist, the viable combinations are much more, making freedom of choice and personalization much more relevant.

It's quite simple, really.