Every time I saw this topic, I just jumped it and didn't cared... but, later, I thought up on some points that you brought up and I'll add my two cents.
So, with the boom of the GE:S being knowable in social media (probably thanks to youtubers and more that played and streamed the game to a lot of people), a bunch of people that doesn't know how this works or why (nameable newbies) entered the game to give a try. Yes, they are some gamemodes (or gameplay scenarios, but I prefer the first name rather than the last) that "requires" skill and others that don't. All games have that. So let's see the mind of a newbie in-game (counting the social media boom)
DEATHMATCH (DM) and TEAM DEATHMATCH (TDM) → Both are simple. You have a gun, shoot some people, the end. The difference is that the second makes teams. When a newbie comes to play it, he'll see the same thing that a lot of games have; what changes is the graphics and the lack of a giant fanbase. Even that's a good way to know more about the guns of the game, it will bore him faster to play the same gamemode almost everything has it.
CAPTURE THE FLAG (CTF) → Same with DM, but the newbie learning the game and what differentiate it from the rest slower than DM. However if the newbie is a person who never played a CTF game, he'll bring hell to the team since he'll get the flag at some moment and run around the map like nothing happened.
GUN TRADE (GT) → Good way to learn about the guns. Though, you snooze, you lose (about the weapon... if you die, then you change weapons)
ARSENAL (ARS?) → Makes the newbie to learn about the guns faster than GT, though since not everyone on the server is new as well, he'll probably just know the 3-4 guns each round (if the loadout doesn't repeat low-level guns).
LICENSE TO KILL (LTK) → It's and easy to kill gamemode, but is frantic. If you don't like exercise, then buckle up: you will need to. The newbie will learn the hard way that being idle is the wrong way of being, making him to learn how to dodge how he can (except his name is Gohan)
MAN WITH GOLDEN GUN → Nice gamemode. Like a King of the Hill, but without the location. Unfortunately newbies will learn the same way as DM since the Golden Gun doesn't have much to give. I know, it's powerful and can one-shot, but if you are a newbie, that's gonna be difficult.
A VIEW TO A KILL (VTAK) → Benefits hiding and campers. So newbies will do the opposite and almost never win. Next!
LIVING DAYLIGHTS (LD) → Newbies, when picking up the flag, they will do the same as CTF. The only good thing (or bad thing; I don't know your POV) is that, upon running around aimlessly won't bring pain to anyone (except himself).
YOU ONLY LIVE TWICE (YOLT) → He'll die firsthand and will be on spectator mode until he gets bored and leaves.
DR. NO ARMOR ("NOOOOOOOO!" -Darth Vader) → Same as DM, but faster dieing and faster "leaving the server"
So...yeah... No kidding that Arsenal and LTK are top of the game now. Besides Deathmatch, they are the best at newbie-learning. But since 4.2 had mostly Deathmatch servers, newbies BEFORE the social media boom started to take on other gamemodes and preferred Arsenal and LTK. And, during/after the social media boom, LTK and Arsenal servers were getting with a lot of people and, in consequence, stronger. So it's understandable.
