It's only in trying to revisit the beloved titles of my youth that I'm finding 'gaps' in what is currently available.

We have two volumes of the 90's GHOST RIDER (Danny Ketch) but none of GR's first foray.

What's up with that?

I've given up trying to make sense of how they decide on which volumes to collect in omnibus format. Sales alone can't be a deciding factor, because a JB Superman omnibus is so sought after it would sell out in a heartbeat, yet it's nowhere to be seen. 

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No, but a Volume 1 would be a start. What annoys me is that they've done an EXILES omnibus, which picks up directly after JB left the Superman books! Now they're doing the "Triangle years". 

I too am trying to collect the classic 60's and 70's (and early 80's) material from my youth.

I gave in and bought some Omnibuses (they're so heavy!) for material that I couldn't get in softcover--
NOVA: RICHARD RIDER
ADAM STRANGE: SILVER AGE OMNIBUS
and there are others available that I haven't bought--
SHE-HULK: THE SAVAGE SHE-HULK

WARLORD BY MIKE GRELL (upcoming)

DOOM PATROL: THE BRONZE AGE

or later volumes (they stopped the softcovers but continue with Omnibuses) of--
MASTER OF KUNG FU
CONAN THE BARBARIAN

and things where the original material is available in neither (color) softcover or Omnibus--

JONAH HEX
SPIDER-WOMAN

I'm not sure why, if the material is worthwhile to collect in an expensive format, why not print it in an inexpensive format as well?  Or vice versa?  Isn't the hard work of scanning, digital reproduction, or other pre-press work already done?

But let me re-phrase my question: Why sell some books ONLY in soft cover and other books ONLY in hard cover?