Doing MDAU fancomic, I thought again about why early Zel fandom (at least Japan and Korea) supported Z/L so much. In short, I think that's partly (or mostly) because the novel story was told in the first person, not in the third person like anime.
In both countries, vol 1-7 of the novel were released before anime. Since the story was told in the first person (Lina is the speaker), novel readers in the early fandom consciously/ unconsciously had identified themselves with Lina.
Back then, many of novel readers were madly in love with Zel. Zelgadiss the white (Shiro-no-Zelgadiss), AKA Death in white from head to toe, with a unexpected warm feel, sometimes showing subtle signs of affection to Lina(->identified with the reader herself). Mr. Kanzaka mentioned "Zel fans are so enthusiastic that they're willing to sell their own houses and invest all those money to buy voting slips for Zel" in mid 90's.
And vol. 7, when the well-known illust of Zel-Amelia arm-in-arm was released, those Zel fandom became a chaos. Japanese Zel fans cried out "please take her off from Zel". This reaction influenced Korean fandom, too. Unlike anime fans, many novel fans were identifying themselves with Lina. Thus, it must be hard to see the coupling concepts without a biased (subjective) view. I guess they must feel like jealousy toward Amelia taking their love away
Before Amelia came out-- In vol. 3, Lina fell into critical condition, attacked by Lantz (who's controlled by Elis Vrumugun). Zel, Gourry & Sylphiel were at other places separately.
When Lina got the attack and blacked out-- Zel felt a cold shiver ran through him, after hearing a subtle metal sound. He immediately dashed to Lina's place, bound Lantz, and stopped the flow of blood from Lina's stomach. But Lina lost too much blood--Zel felt helpless because he didn't have recovery skills. At that time, Gourry and Sylphiel came back--Thanks to Sylphiel's resurrection, Lina could come to herself.
At this time, Mr. Kanzaka felt the need of a main healer in the Slayers party. Sylphiel could not be a main party member, since Gourry-Syphiel would threaten Gourry-Lina. So he introduced a new character, Amelia, in the next volume (vol. 4). Zel learned Recovery from Amelia later.
About the couplings, Mr. Kanzaka interviewed like this: 'Since Lina is Gourry's girl, so I just tried matching leftover characters together'.
That meant, Zel would be out of spotlight with Amelia (and it became true in part 2). That did not please early Zel fandom. Until then, Zel's role had been quite important in the novel. In Korea, Zel fans reacted to make tons of Z/L fanworks, hoping he could return to a main position (The speaker is Lina, so a character who's close to her has more possibility of being at the center of events). In Japan, they were not happy at first, but finally respected Mr. Kanzaka's thought- and found a charm of Z/A, I heard.
After anime released, got popular, ever since the anime fans outnumbered the novel readers, Z/A became a confirmed official. In anime, Z/A looks lovely, and we can see clear signs of affections between the two.
...But I still love the moments of Z/L in the novel (+some of in anime) X3