I'm not sure that is true, I don't remember device support being much better in the past. But assuming that is the case, part of the reason might be the fast evolution of devices and Android itself back in the day. Old devices quickly becoming obsolete and new Android versions being desirable made custom ROMs very attractive for a group of people, including myself.
I installed CyanogenMod on my first smartphone because I could get a more recent Android version when it actually made a difference. As Android evolved, an up-to-date install is not as different anymore than a 3 year-old one, not in features and even less in looks. The main motivations I see for custom ROMs nowadays is to improve your privacy and decrease bloatware, but I don't think most users would bother with going through the process and voiding their warranty to have these things.