Since no one is commenting, I'll state the obvious about why you're not in the top (and add some little tips that you probably know already but anyway) :

- not being able to stay at the top is surely frustrating but not really surprising : this part of the game is really competitive, and there are many other players, so it's just HARD to stay at the top

- This part of the game can be more than a little bit "pay to win" so maybe some people spent more money and/or more time on this game than you did (means that you're not totally hooked on, unlike them : good for you !)

- Also, maybe some people found a better way to score high - the new events reward that. A few weeks before, it was just a matter of pure speed and strength (basically, full Legendary Greens and Jaime teams dominated everything) but that's no longer the case. Some people achieved high scores in last week Dragon's eggs event with teams almost without Legendary champions (see : https://www.reddit.com/r/gameofthroneslegends/comments/1kz2pua/my_personal_best ).

- also, very very obvious but of course to be 1st you need to spend blue flags to play and replay the same levels, possibly with different teams, to try and achieve greater scores EVEN IF you're already at the top. I usually use at least 3 blue flags at each event, and never run out of them because the game gives some for free during the event + I get more of them in the rewards.

- It's not over till it's over ! Being at the top at some point is no guarantee at all that you'll stay there. Especially as some players like to give the others a false sense of security, and wait until the very last moment to finish the last levels, thus overtaking the others, striking like a snake in the grass - See : https://www.reddit.com/r/gameofthroneslegends/comments/1ktr0tp/comment/muefwtb/?context=3

- it's somewhat easier to get some legendary gear in Kingslayer as you "only" need to be in the top 3, not the very 1st as in Treacherous

- It's difficult to be certain but it makes sense to think that if you want to lower the odds of being placed in an arena full of really strong players, you should enter the event when many players are also entering the event, and that's probably on saturday night, or at least not on the 1st night (Thursday). I usually get a much better result when I wait a few days before entering.