Why is it always Sents/Maras?

Because Sentinels and Marauders are the apex class, as evidenced by this video or by this expanded explanation

Its probably something to do with force leap, knights and warriors in general are jump slash, slash see new group jump slash, slash repeat etc for their general solo play (which is 100mph most of the time) they just don't change playstyle to fit group mechanics.

Its probably not surprising that this happens on a class where everyone gets a closer as standard (vanguards only get it as shieldtech although roumers I have heard indicate it may change), guardians/juggs just must be a bit more reserved since they chose 1 saber 2 = flashy.

Funny, I just watched this video titled How to Marauder yesterday. Very funny and it obvious the guy just got tired of running into these types of players so he did a spoof video... funny thing is he never died, even soloed one of the bosses.

/This

You are not a true Marauder/Sentinel unless you pull before tank, wake all the CCs with saber throws and stand in all the AOEs.

Golden rule as a tank or healer - if dps pulls, just don't help him. When he can manage by himself then he can do it all the time.

How to marauder.

https://www.youtube.com/watch?v=f_H48u6kxnw

I'm not defending the behaviour, and I realise this doesn't explain why them and not Guardians, but I wonder if it's partly because they regenerate their resource in combat, rather than out of it. The Sentinel who keeps leaping in ahead of the tank may be getting fixated on "I've got a full focus bar and stacks of centring that are going to dissipate in a moment and I don't want to waste them!" Yes, it's still bad planning; resource management is important to a fragile sentinel, but not that important, however, I think it might be this in some cases; tunnel vision rather than total mindlessness.

lol, that is Awesome!

Oh man, that video too funny hahaha!

I mostly tank flashpoints, and honestly, most playerss, including sents/marauders, wait for me to pull. What they, and other dps typically don't respect is the kill order.

With the changes in 4.0 changing most FP's to tacticals, this probably won't matter too much, as tacticals are pretty much a free for all.