Because absence of zones to capture, skirmish leads to 4v4 smash on one spot. Basically, there is always chaos and everyone is beating all around. This is not swords games in those cases, but hack and slash on 10 square meters. Also, 1vMANY is much more common in skirmish. Skirmish is actually for people who suck in this game.
Honestly, I hate any game mode that includes boosts. Gear stats already break the game, but all it takes is a PK to grab a damage boost with her superior speed at max gear level and it's beyond broken and simply anti-fun, doing half your health bar with a single lunge attack.
Basically, Skirmish is trash because it's a gank-fest by design that also includes a system designed inherently to make matchups imbalanced, besides the enormous imbalance that already exists from gear score. It's basically Dominion, without the strategy and designed solely to make bad Assassins think they're good.
I wouldn't go so far as to say it's a mode for people who suck, but it is a mode that expects weird things from you.
Lacking the Objectives of Dominion, you have a decentralized fight system where battles "could" happen anywhere. Because of the nature with the experience system, and the ease of healing from an assist (feat), you end up with two roaming packs of fighters. It's typically won by whichever team successful ganks the first guy, and starts camping the corpse.
You also have the problem of AI. In no other system is the AI so much of a liability. You have to keep using (LB, RB, Y) "Group Up", or you just watch your AI ally feed the other team 4v1. This is also a problem with PC's, but they should know better.
Then you have the Medium Creeps, god I hate these guys. There's too little benefit to killing them, and no benefit to assisting the kill. They occupy space messing up pathing, frequently get in the way of killing blows, disrupt your attacks, and occasionally block! If you spend time killing them, you're losing momentum, time, and a lot of endurance. They're very annoying speed bumps.
The only unique ability to skirmish is the Veteran upgrade which lets you have full skills temporarily. To get it you have to run off the beaten path, and have specialized in aggressive feats. Even if you've specced your gear for Feat reduction, it's very hard to find the enemy, and make use of multiple skills before it runs out. You've also left your team to fight 4v3 for 15-20 secs, more then enough time for them to down one.
They don't share Orders with Elimination, which is a shame. Getting 50 takedowns in Elimination can take 5+ games, but you could do the same thing in 3 in Skirmish. Again, less experience, more time.
Finally, how do you spec a character for Skirmish? There's less downtime in Skirmish so Regen out of combat is silly, but there's no centralized healing so you really need healing. Same with Classes, a Conq in Dominion is awesome, can be good in Elimination, Skirmish? All you need to specialize in is ganking.
The mode takes a long time to win, rewards spamming attacks, encourages unequal fights, and doesn't reward enough.
Why do you like the mode?
So, I have 4 orders for elimination, no one for skirmish, elimination is set as preferred mode, but I was just matchmaked 4 times in a row into skirmish. This is why I hate skirmish. Stupid mode, that I hate, which will not help me to complete my orders, but I'm still being forced to play it