In COA ssin has a higher chance to get into parties compared to rogue, as rogue is dependent on using pots for the dps, but when fighting Aromine, nobody can use pots otherwise will wipe.
Well there is that, but another reason as to why Sins were preferred was also because back then people kept thinking rogue "used the yellow token" (shared with knight/sorc/ranger). Most of them preferred to take the assassin (blue), over the rogue ("yellow"*). Of course that was not true though
*I quote "yellow" because rogues actually need the "red token". A simple misunderstanding turned out to be a fatal burnout for those who chose rogue on the SEA version.
Another probable reason is due to [Shadow Armor] vs [Gangster Paradise] on situation-al sequences. For example;
- raid boss is about to dish out a 10k unavoidable AOE attack
- Both the rogue and assassin have 8k max hp
- Although [Gangster Paradise] can heal great amounts of hp, they will still die to said AOE
- As opposed to that, the assassin, can just pull a [Shadow Armor] and only take ~5k damage and still live the attack.