Having been a full time Corsair in the past, several reasons why they are a rare sight include but are not limited to:
1) It's expensive (Food, Bullets, Tools, Cards, Numerous sets, etc.)
2) It can be somewhat challenging in that you have multiple duties including keeping tabs on several buffs for two separate groups, pulling, DPS, debuff enhancement with QD, sleep/dispel, etc.
Sadly most fools can barely keep two rolls up per group let alone do decent damage or dispel/sleep or pull in a timely manner. Don't let anyone tell you it's cake to do it all at once.
3)Many don't understand the concept of gambling and taking risk for the sake of bettering your group. Too many fools simply stop at 6 because they're too chicken ***to take the chance to hit the jackpot.
It helps to have snake eye or fold capped, but sometimes there is simply no excuse to live with mediocrity every roll. If you want static effects play bard. At least that way you can use elegy and pull more efficiently.
4) Generally BRD is your bread and butter buff job. They simply bring more to the table such as elegy, faster recasts, more gear suitable for backup heals if not /nin, haste without affecting TP gain, and more. It's hard go wrong with a bard, but there's plenty to lose if your COR can't step up to duty.
However, this by no means should discourage you from playing perhaps one of the most useful and fun to play classes in the game.
I mean who wouldn't wanna sling guns, roll the dice, drink rum and say Arrrggh!?!
Motivation: