I guess the reason why people prefer diurnal rather than nocturnal is that you need to know/pay attention to the incoming mechanics to mitigate...which I guess a lot of people does not prefer to? But as AST main I prefer going with Nocturnal cause I find it more enjoyable mitigating between playing yu gi oh cards and dps-ing while the rest is up to Earthly Star (why do most of AST forget that earthly star exists anyway? that skill is the love of my life) and few occasional helios since my WHM partner can prolly cover the rest with a flick of a hand. Moreover, because in my server SCH tends to never shield so I'm really grateful whenever I'm partnered with a WHM. Whenever I go diurnal, I just know that I'm going to work extra hard to cover the mitigation-less party, that's how bad SCH players are in my server, though some are hella jaw-dropping awesome, most of em are just meh.
Plus, I'm playing on the Japanese server, so communication is hardly an option.