It seems almost odd that I have to have a rationale for Lilah as a slashy character. She's quite possibly the most canonically bisexual of any Buffyverse character between picking Faith up in a bar in "Five by Five," flirting heavily with her charge Bethany in "Untouched," and smirkingly taunting Cordelia about being turned on in "Calvary" (which, in light of Cordelia stabbing her in the neck about twenty minutes later, might have been a mistake). Furthermore, Drusilla (another fairly slashy character in the Buffyverse) takes immediately to Lilah and her beautiful skin in "Reunion." So that makes four characters with the homoerotic UST with Ms. Morgan. So canonically, it's odd to have to justify Lilahslash.
At the same time, the majority of attention to Lilah has been because of Lilah/Wesley, with Lilah/Angel and Lilah/Lindsey coming in second and third as most-written Lilah ships. The early tendency towards subtextually implying that Lilah is a lipstick lesbian has more or less been erased by "Carpe Noctem." Yet at the same time, Mutant Enemy still stuck in the Lilah/Cordy moment in "Calvary" and gave us almost-canonically-bisexual Lilah, who should be as easy to slash as Faith. And yet, she isn't.
Why?
First of all, one can't get past the part where Lilah/Wes has defined Lilah as a character. If it's the only true thing she's ever felt (though that was Wes talking through Lilah), it doesn't bode well for a lengthy epic where Cordelia and Lilah are soulmates. Then again, when has that stopped any other pairing? Sure, the whole "true love forever" tends to spawn heartier ships, but Lilah is a fun character to ship. She's kinky, she's overtly sexual, and one gets the feeling that she'll do anything that moves, even if canon seems to suggest the opposite. At the same time, Lilah/Wes has also allowed Lilah to be more than the cliche bad girl. He makes her complicated, she makes him complicated, and it's a tough dynamic to give up on when it's canon, hot, and interesting. Especially when you're slashing Lilah with characters she hasn't met or she doesn't get along with terribly well.
Also, there's the question of the other female character. Not to put too fine a point on it, but Lilah tends to piss everyone off. She has the great sexual chemistry with Faith; she also tried to have Faith killed. She and Cordelia seem to have similar personalities and a certain level of crackle; but yes, she also tried to have Cordelia maimed/killed. There's Darla, who Lilah wanted dead, so again with the hard. Drusilla seemed to like Lilah, but Dru is not really Lilah's type. Fred and Lilah--well, snicker. The reasons why a good organic Fred/Lilah is difficult outnumber why it's easy. (I maintain that Fred and Lilah protest their hatred too much and one day, the boys will find them screwing on a desk, but that's neither here nor there). Lilah hasn't met Gwen, Buffy, Dawn, Willow, Anya, et cetera, and one imagines that Lilah and Buffy in particular would try to kill each other. Literally. Thus, to write Lilahslash, one either has to write an OFC or one has to deal with the part where Lilah's a total bitch to other women. And that makes writing frothy PWPs harder to imagine. You can't just think, "hey, I'll write me a Fred/Lilah, cuz they're so pretty!" you have to consider why they would, how they would, and if you're willing to deal with their true and abiding hate for each other.
Finally, there's the big thing: the mystery of Lilah the character. I feel like I go over this a whole lot, but even Stephanie Romanov has admitted for the great majority of Lilah's tenure on the show, she's basically done the same scene over and over. We don't know Lilah very well. And what we've seen of her is not terribly flattering. So while it's easy to think of Lilahslash, it's a question of "well, why do you want to when she's not sympathetic unless she's with Wes?"
I don't have a good answer for that. I mean, there's clearly some smoking hot antagonistic sex that can be written for Lilah, slash or not. A lack of backstory canonically means you have a lot of free play that you don't for other characters. And given the general lack of onscreen chemistry among Angel characters, it seems odd that we'd want to give up a character who's got it in spades just because she's harder to write.
In any case, Lilahslash. We need more of it, it makes a lot of sense, and it would be smoking hot. So consider it, dammit.