Lucy/Gray gained a following because they're the straight men of the group, so it seems like they have more in common.
Lucy/Loke gained a following because he flirts with her whenever he makes an appearance, and also that stuff where she saved his life.
Lucy/Sting gained a following because a couple people who were toying with the idea did a good job writing fanfics or drawing them together and other people thought it looked cute. Kinda weird how his laughing didn't really sink that pair but everyone abandon Sting/Yukino just because he wasn't fazed by her getting kicked out.
I dunno where you read your Fairy Tail fanfics but 95% of fanfics are usually teens writing about cheating and stupid drama to capture readers and sadly it works. Leading to others copying them and fanfics being very terrible writing that gets praise from others with little to no originality.
There is of course the 5% that are decent if not great but it seems the percentage is getting even smaller.
I read most Fairy Tail fanfics on Fanfiction and 90% of shipping fics that deals with Natsu and Lucy involves Lisanna in some way. Whether its Natsu kicking Lucy off the team or the jealously angle. Its frankly terrible and I should stop torturing myself but the 5% of good fics keep bringing me back. The good news is that a fanfic's summary is a good indication of which ones to avoid.
Putting my rant aside about the terrible state of fanfics, specifically shipping ones.
I have to agree with you on the Sting and Lucy ship actually sailing.
Maybe the Natsu/Lisanna ship them to get rid of Lucy or just Natsu/Lucy haters.
I personally support Natsu/Lucy for those that haven't figured it out but I can see why Gray/Lucy and Loke/Lucy is popular. I just cannot see how or why Sting/Lucy
is popular at this early stage.
Even Rouge/Levy is getting popular.
Its like people ship them for the sake of making sure the 1st gen dragon slayers don't end up with them. >.>^
I just hope Sting/Yukino make a comeback so I can deal with less Sting/Lucy or Natsu/Lucy becomes official. Although becoming official rarely affects shipping in fanfics as shippers are more likely to ship the opposite pairing even harder as is the case with Harry/Herminone in Harry Potter section.
This is just my opinion of course. If you ship Sting/Lucy or Rouge/Levy please don't take offense.
BTW some slightly humorous news for the shippers here. There is actually a Natsu/Minerva pairing on Fanfic.net lol. Talk about crack pairings.