I don't believe there is a common denominator among backers, nor do I think people are idiots. Instead there are a number of things that each grab a subset of people. Examples but not limited to just them.
a) Astronaut dream - walking on a space ship, landing on the moon and exploring the stars - Star Citizen has the virtual version of that dream with the promise of more
b) Panini sticker collector (complete collections and FOMO) - remember collecting football stickers and the desire to complete the set - here is the adult version - but with an ever expanding collection
c) Single system, long travel times and no solo mode - keeping the player base concentrated
d) Viewing a game as a hobby or a second (or maybe even "real") life
e) Regular videos and bugs - There is always new entertainment to talk about fostering a) to d)
f) Perpetual development with no full release maintaining a) to e)
I remember first playing the original Elite it felt like I was living my astronaut dream but only a subset of it - SC has a) enough game to provide the essence of that dream but always promising much more. I collected Panini stickers a couple of times and it taught me how addictive it can be and I learned a valuable life lesson, but if you never had that lesson this can be mightily addictive. c) fosters players talking to one another and playing with friends.
Gaming for me is something I do when I have time, but I think for some it is very much more than that and depending how far you take that it's easy to justify significant spending. Given how long this has been going for three grand a year on your second life/hobby would by now get you close to a Legatus pack.
The last one is the most important because there is no reality check - you can always believe the next patch will be better and eventually they will get there. The invested optimist always believes things will get better.
In my view it's been a very carefully curated experience designed to appeal to an array of mindsets and it has been staggeringly successful.