Sora no Kiseki/Trails in the Sky SC is Falcom's only game that is
not on PSN. This is because of the game being over two UMDs, and requiring to switch UMD's as you move about Liberl. In order to make it a digital only release, it would probably require reprogramming the game completely to make it fit into the required UMD size limit.
This IS possible, because Zero no Kiseki is about the same size as SC, and is using a new compression utility that Falcom developed... then Ao no Kiseki is
significantly larger than Zero. Both of these games are on a single UMD. ...but this may mean that Falcom might have to do the programming, since I don't know (I don't think) XSEED has the resources to do something of that sort.
If you want proof of the script sizes, Falcom published three books, each of which having the script to both the main scenario and the side quests printed in them. (this doesn't include NPC's or items, or other sorts of text, as a note.)
I've posted a photo of the books next to each with their page counts:
http://twitpic.com/909qkw
Only the voices were added to the PC version of FC. There are other things that were added that weren't to the Win7 PC version. These are things like the difficulty modes, and so forth. I also think the retry offset that made battles easier in FC is a PSP-only thing, as well. (Don't quote me on that, I don't know for sure off the top of my head.)
It gets more interesting when you get to SC and 3rd, however. There are entire playable characters added to the PSP version of SC, new items, difficulty modes, and so forth.
Then 3rd has several major additions to it (along with something removed as well, but that's a long and spoilery story)- mostly in the form of new quartz and whatnot.