I don't think it's just a matter of how things are marketed or how big the existing fanbase is. Kino's Journey has a dedicated fanbase,
and Tokyopop's release of it had a redesigned, 'retail friendly' cover, yet it did not sell well. On the flip side of things, Yen's Book Girl seems to be doing well enough. Volume 3 will be released soon, and they've scheduled release dates for the entire series on their website- excluding the sidestories- which I'm assuming is a good sign. Yet, at the time they licensed it, its fanbase was practically non-existant; if I remember correctly, the anime movie was barely in theatres in Japan back then. The cover is redesigned, but still has the manga-style illustration, and the contents retain things that may be offputting to 'outsiders'; the Japanese setting, the fact that some of the plot of the first volume requires at least a little bit of knowledge of Osamu Dazai's work, etc. . So, where is the difference? What makes one sell, while the other does not?
Being, perhaps, a little harsh to other publishers (for which I apologise), I feel that it's also influenced by whoever is picking licenses at Yen Press has better intuition when it comes to what's worth taking a chance on.
Looking at their releases, and ignoring Haruhi and Spice and Wolf for a minute, they're all finished series that are relatively short (<10 volumes) as far as light novels seem to go. S&W is a little longer, but it is also complete- excluding the recently announced epilogue novel, but I think it's safe to assume there won't be more after that. Personally, I find it hard to buy ongoing things if there's no guarantee of seeing the next part. I doubt I'm alone in this. So, knowing that, I can see why they're going for that kind of thing; the shorter the series, the easier it is to commit to releasing all of it, especially if it's complete. As a buyer, I'd rather purchase the first volume of a series like that than something with 20+ volumes that I'm not sure I'll ever get to read most of.
I have to agree with you somewhat on the 'not being marketed as light novels' front, especially in regards to S&W and Kieli, which are already pretty 'English friendly' to begin with. As for the cover redesigns, I really wonder whether they help or hinder; they seem to alienate existing fans, but can be good for bringing in new ones.
Anyway, I think I've typed far too much, and it's far too early in the morning for me, so I'm probably digressing from the original point; especially since I've gone ahead and said 'I'd love to see an English release of the Index novels!' and then happily pointed out why it wouldn't be good business sense. xD
It's a shame, really. I do my best to support light novel releases (I've never seen the anime of Strawberry Panic, but I'll be picking up the novels once I get paid this month simply because of that), but it seems that everyone is too scared to take risks, and those who do rarely have much luck with their gamble.
I really hope to see more LN releases in the future.
Oh, and on a slightly related note; if I were a publisher, I'd snap up Bakemonogatari. It has a sizeable fanbase from the anime adaptation, will likely pick up more if the hints of a North American release of that are correct, and while it's technically 12 volumes long, each 'part' (Kizumonogatari, Bakemonogatari, Nisemonogatari, etc.) is short enough that the problem of people hesitating to buy because of the possibility of no conclusion could easily be avoided. Winkwinknudgenudge. /blatant pleading.