Because tanks only need 3 buttons to be efficient 99% of the game, healers only need 2 or 3 depending if it's aoe or not (and usually have one dps spell anyway), while dps have triple if not five times the buttons to press, positionals, specific cds and more that healers and tanks usually don't need to care about for most of the game. Talk about fairness, am I right?
With that said, I don't know how much BLM was changed, but don't forget that blm can be tough with certain bosses and if they require moving a lot, you lose dps and sometimes your buffs. But I haven't touched it yet and I'm still unaware of the changes: I can only speak from a former HW BLM
Honestly the best advice I can give is to actually check online for guides, use the SSS or whatever simulator to check your dps (If you're anti-parser that is) and see what gets better and what not. Also try to get into some linkshells or guilds or whatever because DPS are usually the most ignored, since they are so many they are basically "cheap products". Being in more groups will help you find the good rotation.
