that's becasue holding threat on Sin is incredibly easy (plus stealth). but given that their mitigation is nearly all of active variety.. that makes them very challenging to play well
juggs incidentally are opposite. you are going to have to work very hard for your threat, but will be nearly unkillable. with some pretty awesome cooldowns to boot. - having played all three, in order of personal preference, I'd rate them: sin>powertech>jugg
Except in NiM where Juggs not only have to work the hardest for threat (both aoe and single target), without excruciatingly good cooldown usage, they lag behind sin's and PT's significantly in damage taken. (A sin tank's defensive cooldown usage is much more straightforward than a jug's)
PT is the lol@effort tank, but has relatively little to bring to the table unique to other classes. (its kinda Meh at everything unless hydraulic override provides some benefit)
Sin is the lol@Mechanics tank, but with a few exceptions takes the most spike damage, and is truly awful in some cases, while being god incarnate in others. Sin also has by far the most rigid rotation of any tank class (if you want to keep max mitigation), and in some cases quite literally cannot keep up its 4% DR. It also does the least self healing out of all the classes
Juggs are kinda in-between the two in both effort and cheese-ability (read as saber reflect
). The biggest issue juggs have is they have some of the hardest time generating starter threat (although if you can use saber reflect at the beginning then again its very comparable, and even so its not that hard), and for real progression raiding (more than any other class) you really need to know when to use your cool-downs (since saber ward does almost nothing on some fights and saber reflect does less on others).
Juggs also suffer heavily relative to the other two in movement ability and at range damage.
That being said, I still like jug tanking the most (and surprisingly they actually do have the highest top-end aoe dps), but it really is a matter of personal preference.