It's because they wanted to get rid of the fact that SKs and Pallies were Crusaders and tossed them by themselves. While an Assassin is a "rogue" in the same sense that Swashbucklers and Brigands both are melee-combat based scouts.
__________________
Everyday in the week i'm in a different city
If i stay too long people try to pull me down
They talk about me like a dog
Talkin' about the clothes i wear
But they don't realise they're the ones who's square
Hey! And that's why
You can't hold me down
I don't want to be down i gotta move Hey!
They grouped them according to a more EQ1 relationship. In EQ1, Assassins, Brigand and Swashie were all Rogues. While a Ranger was a class to themselves.