Any reasons why Dark Elves getting that much worse?
Short answer: woodelves. Others shine depending on situation. Low TV armored elves are quite good, as the gap in bashing capabilities between elves and anything bashy elsewhere is not that huge. Late deep throwing elves are better as you cannot reasonably expect to trade blows with anything and come on top. Low to mid TV inbuilt dodge is also important before everyonehas tackle to aim for attrition.
Welves just check most of these boxes, with wardancers being on top, offering ball sacking chance even for idiots. Union is not as bad as it may seem, but requires better knoledge of the game, as your survivability is worse than welves with tricks relevant only on sky high TVs.
I'ts not so much that they get worse, its that the other teams get better.
A developed high elf team has a nearly unstoppable offense. They can play lines just as well as other elf teams on defense. Pro elves just start with so many skills, by the time you fill up with positionals and level them, you have so many options is ridiculous. And the woodies will always have speed, something that is ALWAYS useful at any tv.
I disagree about woodies being the best at low tv. Their fragility and cost makes them really easy to get a numbers advantage on. A single secret weapon can counter the wardancers and leave the team without much offense. At mid tv they can compensate for this, but they start out rough
Most tournaments are played with res rules, which favors wood elves heavily. It's not surprising that a tournament with rules that favor a race see that race perform well...
Res style tournaments != league play
Skaven, norse, and amazon likewise do better at tournies than they do in leagues, but that is probably just coincidence that its the 7 av teams, right?
Alrthough with ladder leagues like the official ones, the ability to restart your team when the first couple games go poorly probably makes squishy teams more attractive...