timing is a factor, but not a huuuuuge factor. people still want white ravens. the problem is that rivenroad (hard) is designed in such a way as to have very strict requirements as far as tanking, healing and dps... and given that it takes most LSs almost the full 30 minutes to actually score a kill, it feels very punishing and unforgiving when those requirements aren't met and you wipe. (ie: you do everything right and don't blow yourself up but still don't meet the DPS requirements and have to start all over again from scratch)

ifrit extreme is harder overall, i think, due to latency. but as far as straight up battle design goes rivenroad (hard) is much more execution-heavy and most people just don't have the gear/skill/patience to try a 30 minute fight over and over again until they finally get it right.

as far as achievement tracking on xivcore goes, it looks like only about 10% of registered members have completed garuda. less have completed ifrit extreme (i'm gonna estimate 3-5%)... and only 1% have finished rivenroad hard.

so, yeah. tl;dr i think it's a combination of the fight being too hard for the current player base as well as the current player base largely lacking the fortitude to gut it out until they win.