Well I don't know if I made it clear. But basically, tickets to the Chemical Brothers concert went on sale last year. As I remember it, before Field Day went on sale, before Good Vibes was announced, before Future Music Fest was announced.
The Chems solo concert basically went on sale in a time when festival season was just kicking off to a massive start. People had money, they were enthusiastic about spending it on festival tickets.
Now, we are well into festival season. Future Music Fest is now on sale at a time when people are starting to feel "partied out" (December alone could have done this), or at least "ticket paying for'ed out".
It seems that this year people assume every event will sell out almost instantly. While last year, as I remember it, most major festivals didn't sell out until within weeks of them actually being.
And yea, I'm sure plenty of Chems Brothers fans bought tickets to the solo gig when they went on sale, and those people probably wouldn't justify paying for FMF tix to see them again.