Sony is able to publish the game more easily because the PlayStation version of it was published by Crystal Dynamics back in the day, whereas the PC version was published by Activision with Crystal Dynamics' involvement, which led to the legal hell the PC version of the game still dwells in, today. Though the North-American PSN re-release was handled by Sony Computer Entertainment of America, the European version is published by Square-Enix, formerly Eidos, owners of Crystal Dynamics, who fully own the PS BO: LoK's rights. Re-releasing on PC is a lot harder, due to Activision's involvement in that version.
As to the game itself, though I would definitely welcome it, it's definitely not my favourite of the series. Silicon Knights were overrated as hell, they made a few great games that no one played and then went downhill. Blood Omen is great, indeed, but after a very tiring and frustrating campaign to try and fund a "spiritual successor" to Eternal Darkness, I have to say I'm a bit tired of Denis Dyack's obsession with all things lovecraftian, lovecraftian-horror and all that jazz, a feeling that reflects itself in Blood Omen, as well. In all honesty, I very much prefer the direction Amy Hennig took the series with the first Soul Reaver title, ditching the whole pseudo-lovecraftian crap Dyack keeps trying to push in his games, and creating a lore and mythology that took what was started in Blood Omen to a whole new level. I always liked Raziel more than Kain, and I very much prefer the Kain design they used in the Soul Reaver series and Defiance over the bland "vampire anti-hero" design he had in Blood Omen and, to an extent, Blood Omen 2.
Like I said, it would be nice if GOG managed to get the game that started one of the best video game series in the history of gaming, it makes perfect sense, Blood Omen is, without a doubt, a Good Old Game. But is it essential? In my opinion, no, it isn't. What they did from Soul Reaver onwards is far better than what Blood Omen provided, and as far as I'm concerned, GOG has the Legacy of Kain titles that matter the most.