Talent.

The main difference is that in Classic Soul nobody deluded themselves that they could do everything - a great singer did not necessarily mean a great writer. So Classic Soul was
essentially a collaborative effort with many people playing a part, singers, musicians, writers, arrangers, producers and engineers who all brought their skills together to produce a record.
Nowadays because of the advent of computerised technology too many people think they're qualified to do everything when they're not resulting in masses of barely competent records
where you can have somebody with a really good voice but terrible material to work with. How much of this is down to greed is debatable - the "why should I pay somebody else to
arrange the song when I can do it myself" scenario. The other thing is the lack of an objective viewpoint - it's never a bad thing to have another opinion even if being told "well, that
could be better" doesn't stroke the ego as much as listening to the playback of a track and thinking you're God's gift.