Yeah, yeah, a great load of vehicles from GTA IV are missing, it's really sad that they aren't in, but what can you do...
I really hope that the reason they are lacking is technical like what John Footpenis alluded to instead of some silly reason like "they didn't fit the Cali vibe like the Stratum and Premier do"
I was dissapointed in the lack of "everyday" cars like normal sedans, etc. Almost all the sedans were either luxury/exotic sedans or vintage/beater sedans.
Yeah, about that... I don't know for sure, but I did hear that there's some line of code in GTA IV labeled "boring cars" that made cars like the Fortune, Uranus, Hakumai, and of course the Willard spawn less often that they should. Seems R* has a thing against the run of the mill, even though I think they add a much needed middle ground between old clunkers and top of the shelf.
Maybe we'll see them added to the car rotation if they ever incorporate LC to GTA:O, but I wouldn't bet on it